Ship

ABSTRACT

A ship including a storage tank for storing a liquefied gas includes: a boil-off gas (BOG) heat exchanger installed on a downstream of a storage tank and heat-exchanges a compressed BOG (“a first fluid”) by a BOG discharged from the storage tank as a refrigerant, to cool the BOG; a compressor installed on a downstream of the BOG heat exchanger and compresses a part of the BOG discharged from the storage tank; an extra compressor installed on a downstream of the BOG heat exchanger and in parallel with the compressor and compresses the other part of the BOG discharged from the storage tank; a refrigerant heat exchanger which additionally cools the first fluid cooled by the BOG heat exchanger; and a refrigerant decompressing device which expands a second fluid sent to the refrigerant heat exchanger, and then sends the second fluid back to the refrigerant heat exchanger.

TECHNICAL FIELD

The present invention relates to a ship, and more particularly, to aship including a system for re-liquefying boil-off gas left after beingused as fuel of an engine among boil-off gases generated in a storagetank.

BACKGROUND ART

In recent years, consumption of liquefied gas such as liquefied naturalgas (LNG) has been rapidly increasing worldwide. Since a volume ofliquefied gas obtained by liquefying gas at a low temperature is muchsmaller than that of gas, the liquefied gas has an advantage of beingable to increase storage and transport efficiency. In addition, theliquefied gas, including liquefied natural gas, can remove or reduce airpollutants during the liquefaction process, and therefore may also beconsidered as eco-friendly fuel with less emission of air pollutantsduring combustion.

The liquefied natural gas is a colorless transparent liquid obtained bycooling and liquefying methane-based natural gas to about −162° C., andhas about 1/600 less volume than that of natural gas. Therefore, to veryefficiently transport the natural gas, the natural gas needs to beliquefied and transported.

However, since the liquefaction temperature of the natural gas is acryogenic temperature of −162° C. at normal pressure, the liquefiednatural gas is sensitive to temperature change and easily boiled-off. Asa result, the storage tank storing the liquefied natural gas issubjected to a heat insulating process. However, since external heat iscontinuously sent to the storage tank, boil-off gas (BOG) is generatedas the liquefied natural gas is continuously vaporized naturally in thestorage tank during transportation of the liquefied natural gas. Thisgoes the same for other low-temperature liquefied gases such as ethane.

The boil-off gas is a kind of loss and is an important problem intransportation efficiency. In addition, if the boil-off gas isaccumulated in the storage tank, an internal pressure of the tank mayrise excessively, and if the internal pressure of the tank becomes moresevere, the tank is highly likely to be damaged. Accordingly, variousmethods for treating the boil-off gas generated in the storage tank havebeen studied. Recently, to treat the boil-off gas, a method forre-liquefying boil-off gas and returning the re-liquefied boil-off gasto the storage tank, a method for using boil-off gas as an energy sourcefor fuel consumption places like an engine of a ship, or the like havebeen used.

As the method for re-liquefying boil-off gas, there are a method forre-liquefying boil-off gas by heat-exchanging the boil-off gas with arefrigerant by a refrigeration cycle using a separate refrigerant, amethod for re-liquefying boil-off gas by the boil-off gas itself as arefrigerant without using a separate refrigerant, or the like. Inparticular, the system employing the latter method is called a partialre-liquefaction System (PRS).

Generally, on the other hand, as engines which can use natural gas asfuel among engines used for a ship, there are gas fuel engines such as aDFDE engine and an ME-GI engine.

The DFDE engine adopts an Otto cycle which consists of four strokes andinjects natural gas with a relatively low pressure of approximately 6.5bars into a combustion air inlet and compresses the natural gas as thepiston lifts up.

The ME-GI engine adopts a diesel cycle which consists of two strokes andemploys a diesel cycle which directly injects high pressure natural gasnear 300 bars into the combustion chamber around a top dead point of thepiston. Recently, there is a growing interest in the ME-GI engine, whichhas better fuel efficiency and boost efficiency.

DISCLOSURE Technical Problem

An object of the present invention is to provide a ship including asystem capable of providing better boil-off gas re-liquefyingperformance than the existing partial re-liquefaction system.

Technical Solution

According to an exemplary embodiment of the present invention, there isprovided a ship including a storage tank storing liquefied gas,including: a boil-off gas heat exchanger which is installed on adownstream of a storage tank and heat-exchanges and cools a compressedboil-off gas (hereafter referred to as ‘a first fluid’) by a boil-offgas discharged from the storage tank as a refrigerant; a compressorwhich is installed on a downstream of the boil-off gas heat exchangerand compresses a part of the boil-off gas discharged from the storagetank; an extra compressor which is installed on the downstream of theboil-off gas heat exchanger in parallel with the compressor andcompresses the other part of the boil-off gas discharged from thestorage tank; a refrigerant heat exchanger which additionally cools thefirst fluid which is cooled by the boil-off gas heat exchanger; arefrigerant decompressing device which expands a second fluid, which issent to the refrigerant heat exchanger (a fluid sent to the refrigerantheat exchanger hereafter being referred to as ‘a second fluid’) andcooled by the refrigerant heat exchanger, and then sending same back tothe refrigerant heat exchanger; a first decompressing device which isfor expanding the first fluid that is cooled by the boil-off gas heatexchanger and refrigerant heat exchanger; and a third decompressingdevice which expands a part of the first fluid which is cooled by theboil-off gas heat exchanger (hereafter referred to as “a third fluid”)and sends the expanded fluid back to the boil-off gas heat exchanger, inwhich the boil-off gas heat exchanger uses the boil-off gas beingdischarged from the storage tank and the third fluid as a refrigerantand the refrigerant heat exchanger heat exchanges and cools both thefirst fluid and second fluid by the boil-off gas, which passes throughthe refrigerant decompressing device, as a refrigerant, in which thefirst fluid is either the boil-off gas which is compressed by thecompressor or a confluent flow of the boil-off gas compressed by thecompressor and the boil-off gas compressed by the extra compressor, andthe second fluid is either the boil-off gas which is compressed by theextra compressor or a confluent flow of the boil-off gas compressed bythe compressor and the boil-off gas compressed by the extra compressor.

The ship may further include a gas-liquid separator that separates thepartially re-liquefied liquefied gas passing through the boil-off gasheat exchanger, the refrigerant heat exchanger, and the firstdecompressing device and the boil-off gas remaining in a gas phase, inwhich the liquefied gas separated by the gas-liquid separator may besent to the storage tank, and the boil-off gas separated by thegas-liquid separator may be sent to the boil-off gas heat exchanger.

The first fluid may be branched into two flows on an upstream of a highpressure engine, and a part of the first fluid may be sent to theboil-off gas heat exchanger to be cooled and the other part thereof maybe sent to the high pressure engine.

The ship may further include a fourth decompressing device which expandsa third fluid used as a refrigerant in the boil-off gas heat exchanger,in which the boil-off gas passing through the fourth decompressingdevice may be sent to a low pressure engine.

The ship may further include a boost compressor which is installed onthe upstream of the boil-off gas heat exchanger to compress the firstfluid supplied to the boil-off gas heat exchanger.

The boost compressor may have a capacity of ½ relative to that of thecompressor.

The second fluid which is compressed by the extra compressor, passesthrough the refrigerant heat exchanger and the refrigerant decompressingdevice, and then used as the refrigerant of the refrigerant heatexchanger may be sent back to the extra compressor to form a refrigerantcycle of a closed loop in which the extra compressor, the refrigerantheat exchanger, the refrigerant decompressing device, and therefrigerant heat exchanger are connected.

The second fluid which is compressed by the extra compressor, passesthrough the refrigerant heat exchanger and the refrigerant decompressingdevice, and then used as the refrigerant of the refrigerant heatexchanger may be discharged from the storage tank and then joined withthe boil-off gas passing the boil-off gas heat exchanger.

The ship may further include a valve which is installed on a lineconnecting the boil-off gas compressed by the compressor with theboil-off gas compressed by the extra compressor, in which the valve maybe opened and closed to join the boil-off gas compressed by thecompressor with the boil-off gas compressed by the extra compressor orseparate the boil-off gas compressed by the compressor from the boil-offgas compressed by the extra compressor.

The refrigerant decompressing device may be an expander, and the fluidjust before passing through the refrigerant decompressing device and thefluid just after passing through the refrigerant decompressing devicemay be a gas phase.

According to another exemplary embodiment of the present invention,there is provided a boil-off gas treatment system for a ship including astorage tank for storing a liquefied gas, including: a first supply linealong which boil-off gas, which is discharged from the storage tank andpartially compressed by a compressor, is sent to a high pressure engine;a second supply line which is branched from the first supply line andhas an extra compressor provided thereon, with the extra compressorcompressing the other part of the boil-off gas discharged from thestorage tank; a return line which is branched from the first supply lineand has a boil-off gas heat exchanger, a refrigerant heat exchanger, anda first decompressing device provided thereon, with the compressedboil-off gas being re-liquefied by passing through the boil-off gas heatexchanger, the refrigerant heat exchanger, and the first decompressingdevice; a recirculation line which has the refrigerant heat exchangerand a refrigerant decompressing device provided thereon, with theboil-off gas, which is cooled by passing through the refrigerant heatexchanger and a refrigerant decompressing device, being sent back to therefrigerant heat exchanger to be used as a refrigerant and then joinedwith the boil-off gas discharged from the storage tank; a firstadditional line which connects between the recirculation line on adownstream of the refrigerant decompressing device and the refrigerantheat exchanger and a second supply line on an upstream of the extracompressor; a third supply line which is branched from the return lineon the downstream of the boil-off gas heat exchanger, the boil-off gasbeing sent to the low pressure engine by passing through a thirddecompressing device, the boil-off gas heat exchanger and a fourthdecompressing device along the third supply line, in which the boil-offgas heat exchanger heat-exchanges and cools the boil-off gas suppliedalong the return line by the boil-off gas discharged from the storagetank and the boil-off gas supplied along the third supply line as therefrigerant, and the refrigerant heat exchanger heat-exchanges and coolsboth of the boil-off gas supplied along the recirculation line and theboil-off gas supplied along the return line by the boil-off gas passingthrough the refrigerant decompressing device as the refrigerant.

The boil-off gas treatment system may further include a boost compressorwhich is installed on the return line on the upstream of the boil-offgas heat exchanger to compress the boil-off gas.

The boil-off gas treatment system for a ship may further include: afirst valve which is installed on an upstream of the compressor on thefirst supply line; a second valve which is installed on a downstream ofthe compressor on the first supply line; a third valve which isinstalled on an upstream of the extra compressor on the second supplyline; a fourth valve installed on a downstream of the extra compressoron the second supply line; a sixth valve which is provided between thefirst supply line and the second supply line on the recirculation linealong which the boil-off gas branched from the first supply line is sentto the refrigerant heat exchanger; a ninth valve which is installed onthe recirculation line for sending the boil-off gas from the refrigerantheat exchanger to the first supply line; a tenth valve which isinstalled on the first additional line; and a thirteenth valve which isinstalled on the recirculation line between the second supply line andthe refrigerant heat exchanger.

The system may be operated while the first valve, the second valve, thethird valve, the fourth valve, the tenth valve, and the thirteenth valveare open and the sixth valve and the ninth valve are closed, and if theboil-off gas is supplied to the extra compressor, the third valve may beclosed to form the refrigerant cycle of the closed loop in which theboil-off gas circulates the extra compressor, the fourth valve, thethirteenth valve, the refrigerant heat exchanger, the refrigerantdecompressing device, the refrigerant heat exchanger, and the tenthvalve.

The first valve, the second valve, the third valve, the fourth valve,the sixth valve, the ninth valve, and the thirteenth valve may be openand the tenth valve may be closed so that the boil-off gas compressed bythe compressor and the boil-off gas compressed by the extra compressormay be joined and operated.

The first valve, the second valve, the third valve, the fourth valve,the ninth valve, and the thirteenth valve may be open and the sixthvalve and the tenth valve may be closed so that the boil-off gascompressed by the compressor and the boil-off gas compressed by theextra compressor may be separated and operated.

According to another exemplary embodiment of the present invention,there is provided a method including: branching boil-off gas dischargedfrom a liquefied gas storage tank into two and compressing one flow ofthe branched boil-off gas by a compressor; compressing the other flow byan extra compressor; sending at least one of the boil-off gas compressedby the compressor and the boil-off gas compressed by the extracompressor to a high pressure engine or re-liquefying the boil-off gasto return the boil-off gas to the storage tank (hereinafter, referred toas ‘returning boil-off gas’) or re-circulate the boil-off gas(hereinafter, referred to as ‘re-circulated boil-off gas’);heat-exchanging and cooling the ‘returning boil-off gas’ by the boil-offgas discharged from the storage tank and the decompressed and cooledboil-off gas (hereinafter, referred to as ‘decompressed boil-off gas’)as a refrigerant, and then exchanging heat between the heat-exchangedand cooled ‘returning boil-off gas’ and the ‘re-circulated boil-off gas’to be additionally cooled; and cooling and expanding the ‘re-circulatedboil-off gas’ to exchange heat between the cooled and expanded‘re-circulated boil-off gas’ and the ‘returning boil-off gas’.

The ‘decompressed boil-off gas’ may be boil-off gas decompressed after apart of the heat-exchanged and cooled returning boil-off gas' isdecompressed.

The ‘decompressed boil-off gas’ used as the refrigerant for the heatexchange may be sent to the low pressure engine.

The downstream line of the compressor and the downstream line of theextra compressor may be connected to each other to join the boil-off gascompressed by the compressor with the boil-off gas compressed by theextra compressor.

Advantageous Effects

Compared with the existing partial re-liquefaction system (PRS), thepresent invention can increase the re-liquefaction efficiency and there-liquefaction amount since the boil-off gas is decompressed afterundergoing the additional cooling process by the refrigerant heatexchanger. In particular, most or all of the remaining boil-off gas canbe re-liquefied without employing the refrigeration cycle using theseparate refrigerant, and therefore increasing the economicalefficiency.

Further, according to the present invention, it is possible to flexiblycontrol the refrigerant flow rate and the supply of cold heat inresponse to the discharge amount of the boil-off gas, the engine loaddepending on the operating speed of the ship, and the like.

According to the embodiment of the present invention, it is possible tocontribute to securing the space on the ship and save the cost ofadditionally installing the compressor by increasing the re-liquefactionefficiency and the re-liquefaction amount by using the extra compressoralready provided. In particular, the refrigerant heat exchanger can usenot only the boil-off gas compressed by the extra compressor but alsothe boil-off gas compressed by the compressor as the refrigerant toincrease the flow rate of the boil-off gas used as the refrigerant inthe refrigerant heat exchanger, thereby more increasing there-liquefaction efficiency and the re-liquefaction amount.

According to another embodiment of the present invention, thedecompressed boil-off gas sent to the low pressure engine can be used asthe refrigerant in the boil-off gas heat exchanger, thereby increasingthe re-liquefaction efficiency and the re-liquefaction amount.

According to another embodiment of the present invention, the pressureof the boil-off gas undergoing the re-liquefaction process can beincreased due to the additionally included boost compressor, therebyfurther increasing the re-liquefaction efficiency and there-liquefaction amount.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram schematically showing the existingpartial re-liquefaction system.

FIG. 2 is a configuration diagram schematically showing a boil-off gastreatment system for a ship according to a first embodiment of thepresent invention.

FIG. 3 is a configuration diagram schematically showing a boil-off gastreatment system for a ship according to a second embodiment of thepresent invention.

FIG. 4 is a configuration diagram schematically showing a boil-off gastreatment system for a ship according to a third embodiment of thepresent invention.

FIG. 5 is a configuration diagram schematically showing a boil-off gastreatment system for a ship according to a fourth embodiment of thepresent invention.

FIG. 6 is a configuration diagram schematically showing a boil-off gastreatment system for a ship according to a fifth embodiment of thepresent invention.

FIG. 7 is a graph schematically illustrating a phase change of methanedepending on temperature and pressure.

FIG. 8 shows graphs showing temperature values of methane depending on aheat flow under different pressures.

BEST MODE

Hereinafter, configurations and effects of exemplary embodiments of thepresent invention will be described with reference to the accompanyingdrawings. The present invention can variously be applied to ships suchas a ship equipped with an engine using natural gas as fuel and a shipincluding a liquefied gas storage tank. In addition, the followingembodiments may be changed in various forms, and therefore the technicalscope of the present invention is not limited to the followingembodiments.

Boil-off gas systems of the present invention to be described below canbe applied to offshore structures such as LNG FPSO and LNG FSRU, inaddition to all types of ships and offshore structures equipped with astorage tank capable of storing a low-temperature fluid cargo orliquefied gas, i.e., ships such as a liquefied natural gas carrier, aliquefied ethane gas carrier, and LNG RV. However, for convenience ofexplanation, the following embodiments will describe, by way of example,liquefied natural gas which is a typical low-temperature fluid cargo.

Further, a fluid on each line of the present invention may be in any oneof a liquid phase, a gas-liquid mixed state, a gas phase, and asupercritical fluid state, depending on operating conditions of asystem.

FIG. 1 is a configuration diagram schematically showing the existingpartial re-liquefaction system.

Referring to FIG. 1, in the conventional partial re-liquefaction system,the boil-off gas generated and discharged from a storage tank storing afluid cargo is sent along a pipe and compressed by a boil-off gascompressor 10.

A storage tank T is provided with a sealing and heat insulating barrierto be able to store liquefied gas such as liquefied natural gas at acryogenic temperature. However, the sealing and heat insulating barriermay not completely shut off heat transmitted from the outside.Therefore, the liquefied gas is continuously evaporated in the storagetank, so an internal pressure of the storage tank may be increased.Accordingly, to prevent the pressure of the tank from excessivelyincreasing due to the boil-off gas and keep the internal pressure of thetank at an appropriate level, the boil-off gas in the storage tank isdischarged and is then supplied to the boil-off compressor 10.

When the boil-off gas discharged from the storage tank and compressed bythe boil-off gas compressor 10 is referred to as a first stream, thefirst flow of the compressed boil-off gas is divided into a second flowand a third stream, and the second flow may be formed to be liquefiedand then return to the storage tank T, and the third flow may be formedto be supplied to gas fuel consumption places such as a boost engine anda power generation engine in a ship. In this case, in the boil-off gascompressor 10 can compress the boil-off gas to a supply pressure of thefuel consumption place, and the second flow may be branched via all or apart of the boil-off gas compressor if necessary. All of the boil-offgas compressed as the third flow may also be supplied according to theamount of fuel required for the fuel consumption place, and all of thecompressed boil-off gas may return to the storage tank by supplying thewhole amount of boil-off gas as the second stream. An example of the gasfuel consumption places may include a DF generator, a gas turbine, DFDE,and the like, in addition to high pressure gas injection engine (e.g.,ME-GI engines developed by MDT Co., etc.) and low-temperature gasinjection engines (e.g., generation X-dual fuel engine (X-DF engine) byWartsila Co.).

At this time, a heat exchanger 20 is provided to liquefy the second flowof the compressed boil-off gas. The boil-off gas generated from thestorage tank is used as a cold heat supply source of the compressedboil-off gas. The compressed boil-off gas, that is, the second stream,whose temperature rises while being compressed by the boil-off gascompressor while passing through the heat exchanger 20 is cooled, andthe boil-off gas generated from the storage tank and introduced into theheat exchanger 20 is heated and then supplied to the boil-off gascompressor 10.

Since a flow rate of pre-compressed boil-off gas is compressed isgreater than that of the second stream, the second flow of thecompressed boil-off gas may be at least partially liquefied by receivingcold heat from the boil-off gas before being compressed. As describedabove, the heat exchanger exchanges heat the low-temperature boil-offgas immediately after being discharged from the storage tank with thehigh-pressure boil-off gas compressed by the boil-off gas compressor toliquefy the high-pressure boil-off gas.

The boil-off gas of the second flow passing through the heat exchanger20 is further cooled while being decompressed by passing through anexpansion means 30 such as an expansion valve or an expander and is thensupplied to a gas-liquid separator 40. The gas-liquid separator 40separates the liquefied boil-off gas into gas and liquid components. Theliquid component, that is, the liquefied natural gas returns to thestorage tank, and the gas component, that is, the boil-off gas isdischarged from the storage tank to be joined with a flow of boil-offgas supplied to the heat exchanger 20 and the boil-off gas compressor 10or is then supplied back to the heat exchanger 20 to be utilized as acold heat supply source which heat-exchanges high-pressure boil-off gascompressed by the boil-off gas compressor 10. Of course, the boil-offgas may be sent to a gas combustion unit (GCU) or the like to becombusted or may be sent to a gas consumption place (including a gasengine) to be consumed. Another expansion means 50 for additionallydecompressing the gas separated by the gas-liquid separator before beingjoined with the flow of boil-off gas may be further provided.

FIG. 2 is a configuration diagram schematically showing a boil-off gastreatment system for a ship according to a first embodiment of thepresent invention.

Referring to FIG. 2, the system of the present embodiment includes arefrigerant circulator 300 a which is supplied with boil-off gasgenerated from a low temperature fluid cargo stored in a storage tankand circulates the boil-off gas as a refrigerant.

To this end, the system includes a refrigerant supply line CSLa whichsupplies boil-off gas from the storage tank to a refrigerant circulator300 a. The refrigerant supply line is provided with a valve 400 a toshut off the refrigerant supply line CSLa if a sufficient amount ofboil-off gas, which may circulate the refrigerant circulator, issupplied, such that the refrigerant circulator 300 a is operated as aclosed loop.

Similar to the above-described basic embodiment, even in the firstmodified embodiment, the compressor 100 a for compressing the boil-offgas generated from the low-temperature fluid cargo in the storage tank Tis also provided. The boil-off gas generated from the storage tank isintroduced into the compressor 100 a along a boil-off gas supply lineBLa.

The storage tank (T) of the present embodiment may be an independenttype tank in which a load of the fluid cargo is not directly applied toa heat insulating layer, or a membrane type tank in which the load ofthe cargo is directly applied to the heat insulating layer. Theindependent type tank can be used as a pressure vessel which is designedto withstand a pressure of 2 barg or more.

Meanwhile, in the present embodiment, only a line for re-liquefying theboil-off gas is shown. However, the boil-off gas compressed by thecompressor may be supplied as fuel to a fuel consumption place includinga boost engine and a power generation engine of a ship or an offshorestructure. When a ship is anchored, there is little or no consumption ofgas fuel, the whole amount of boil-off gas may also be supplied to are-liquefaction line RLa.

The compressed boil-off gas is supplied to a boil-off gas heat exchanger200 a along the boil-off gas re-liquefaction line RLa. The boil-off gasheat exchanger 200 a is provided over the boil-off gas re-liquefactionline RLa and the boil-off gas supply line BLa to exchange heat betweenboil-off gas introduced into the compressor 100 a and the boil-off gascompressed by at least a part of the compressor. The boil-off gas whosetemperature rises during the compression is cooled through the heatexchange with the low-temperature boil-off gas which is generated fromthe storage tank and is to be introduced into the compressor 100 a.

A downstream of the boil-off gas heat exchanger 200 a is provided with arefrigerant heat exchanger 500 a. The boil-off gas, which is compressedand then heat-exchanged by the boil-off gas heat exchanger isadditionally cooled by the heat exchange with the boil-off gas whichcirculates the refrigerant circulator 300 a.

The refrigerant circulator 300 a includes a refrigerant compressor 310 awhich compresses the boil-off gas supplied from the storage tank, acooler 320 a which cools the boil-off gas compressed by the refrigerantcompressor, and a refrigerant decompressing device 330 a whichdecompresses and additionally cools the boil-off gas cooled by thecooler. The refrigerant decompressing device 330 a may be an expansionvalve or an expander which thermally expands and cools the boil-off gas.

The boil-off gas cooled by the refrigerant decompressing device 330 a issupplied as a refrigerant to the refrigerant heat exchanger 500 a alongthe refrigerant circulation line CCLa. The refrigerant heat exchanger500 a cools the boil-off gas by the heat exchange with the boil-off gassupplied via the boil-off gas heat exchanger 200 a. The boil-off gas ofthe refrigerant circulation line CCLa passing through the refrigerantheat exchanger 500 a is circulated to the refrigerant compressor 310 aand circulates the refrigerant circulation line while undergoing theabove-described compression and cooling processes.

Meanwhile, the boil-off gas of the boil-off gas re-liquefaction line RLacooled by the refrigerant heat exchanger 500 a is decompressed by afirst decompressing device 600 a. The first decompressing device 600 amay be an expansion valve, such as a Joule-Thomson valve, or anexpander.

The decompressed boil-off gas is separated into gas and liquid by beingsupplied to a gas-liquid separator 700 a on a downstream of the firstdecompressing device 600 a, and the liquid separated by the gas-liquidseparator 700 a, that is, the liquefied natural gas is supplied to thestorage tank T and again stored.

The gas separated by the gas-liquid separator 700 a, that is, theboil-off gas is additionally decompressed by a second decompressingdevice 800 a, and is joined with the flow of boil-off gas to beintroduced into the boil-off gas heat exchanger 200 a from the storagetank T or is supplied to the boil-off gas heat exchanger 200 a to beutilized as the cold heat supply source which heat-exchanges ahigh-pressure boil-off gas compressed by the compressor 100 a. Ofcourse, the boil-off gas may be sent to a gas combustion unit (GCU) orthe like to be combusted or may be sent to a fuel consumption place(including a gas engine) to be consumed.

FIG. 3 is a configuration diagram schematically showing a boil-off gastreatment system for a ship according to a second embodiment of thepresent invention.

Referring to FIG. 3, according to the present embodiment, in arefrigerant circulator 300 b, the boil-off gas which is to be introducedinto a refrigerant decompressing device 330 b from a cooler 320 b iscooled by exchanging heat with the boil-off gas decompressed by therefrigerant decompressing device 330 b and then supplied to therefrigerant decompressing device 330 b.

Since the boil-off gas is cooled while being decompressed by therefrigerant decompressing device 330 b, the boil-off gas downstream ofthe refrigerant decompressing device has temperature lower than that ofthe boil-off gas upstream of the refrigerant decompressing device. Inthis regard, according to the present embodiment, the boil-off gasupstream of the refrigerant decompressing device is cooled by exchangingheat with the boil-off gas downstream of the refrigerant decompressingdevice and then introduced into the decompressing device. To this end,as illustrated in FIG. 3, the boil-off gas upstream of the refrigerantdecompressing device 330 b may be supplied to the refrigerant heatexchanger 500 b (portion A of FIG. 3). If necessary, a separate heatexchanging device which may exchange heat between the boil-off gasesupstream and downstream of the refrigerant decompressing device may beadditionally provided.

As described above, the system of the present embodiments can re-liquefyand store the boil-off gas generated from the storage tank fluid cargo,thereby increasing the transportation rate of the fluid cargo. Inparticular, even when the consumption of fuel on the in-ship gasconsumption places is small, the gas can be combusted by the gascombustion unit (GCU) or the like to prevent the pressure of the storagetank from increasing to reduce or eliminate the amount of wasted cargo,thereby preventing a waste of energy.

In addition, the boil-off gas is circulated as the refrigerant to beutilized as the cold heat source for re-liquefaction, therebyeffectively re-liquefying the boil-off gas without configuring theseparate refrigerant cycle, and the separate refrigerant need not besupplied to contribute to securing the in-ship space and increase theeconomical efficiency. In addition, if the refrigerant is insufficientin the refrigerant cycle, the refrigerant may be replenished from thestorage tank to be smoothly replenished and the refrigerant cycle may beeffectively operated.

As described above, the boil-off gas may be re-liquefied by using thecold heat of the boil-off gas itself in multiple steps, so that thesystem configuration for treating the in-ship boil-off gas can besimplified and the cost required to install and operate the apparatusfor complicated boil-off gas treatment can be saved.

FIG. 4 is a configuration diagram schematically showing a boil-off gastreatment system for a ship according to a third embodiment of thepresent invention.

Referring to FIG. 4, the ship of the present embodiment includes: aboil-off gas heat exchanger 110 which is installed on a downstream ofthe storage tank T; a compressor 120 and a first extra compressor 122which are installed on a downstream of the boil-off gas heat exchanger110 to compress boil-off gas discharged from the storage tank T; acooler 130 which lowers temperature of the boil-off gas compressed bythe extra compressor 120; a first extra cooler 132 which lowers thetemperature of the boil-off gas compressed by the first extra compressor122; a first valve 191 which is installed on an upstream of thecompressor 120; a second valve 192 which is installed on a downstream ofthe cooler 130; a third valve 193 which is installed on an upstream ofthe first extra compressor 122; a fourth valve 194 which is installed ona downstream of the first extra cooler 132; a refrigerant heat exchanger140 which additionally cools the boil-off gas cooled by the boil-off gasheat exchanger 110; a refrigerant decompressing device 160 which expandsthe boil-off gas passing through the refrigerant heat exchanger 140 andthen sends the expanded boil-off gas back to the refrigerant heatexchanger 140; and a first decompressing device 150 which expands theboil-off gas additionally cooled by the refrigerant heat exchanger 140.

The boil-off gas, which is naturally generated from the storage tank Tand then discharged, is supplied to a fuel consumption source 180 alongthe first supply line L1. The ship of the present embodiment may furtherinclude an eleventh valve 203 which is installed upstream of the fuelconsumption place 180 to control a flow rate of the boil-off gas sent tothe fuel consumption place 180 and opening/closing thereof.

The boil-off gas heat exchanger 110 is installed on the first supplyline L1 and recovers cold heat from the boil-off gas immediately afterbeing discharged from the storage tank T. The boil-off gas heatexchanger 110 is supplied with the boil-off gas discharged from thestorage tank T and uses the boil-off gas supplied to the boil-off gasheat exchanger 110 along the return line L3 as a refrigerant. The fifthvalve 195 which controls the flow rate of the boil-off gas andopening/closing thereof may be installed on a return line L3.

The compressor 120 and the first extra compressor 122 compress theboil-off gas passing through the boil-off gas heat exchanger 110. Thecompressor 120 is installed on the first supply line L1 and the firstextra compressor 122 is installed on the second supply line L2. Thesecond supply line L2 is branched from the first supply line L1 on theupstream of the compressor 120 and connected to the first supply line L1on the downstream of the compressor 120. In addition, the compressor 120and the first extra compressor 122 are installed in parallel, and mayhave the same performance.

In general, the ship is additionally provided with the first extracompressor 122 and the first extra cooler 132 for preparing for the casewhere the compressor 120 and the cooler 130 fail. Typically, the firstextra compressor 122 and the first extra cooler 132 are not used atordinary times when the compressor 120 or the cooler 130 does not fail.

That is, typically, at ordinary times when the compressor 120 or thecooler 130 does not fail, the third valve 193 on an upstream of thefirst extra compressor 122 and the fourth valve 194 on a downstream ofthe first extra cooler 132 are closed so that the boil-off gas issupplied to the fuel consumption place 180 via the compressor 120 andthe cooler 130, and when the compressor 120 or the cooler 130 fails, thethird valve 193 on the upstream of the first extra compressor 122 andthe fourth valve 194 on the downstream of the first extra cooler 132 areopen and the first valve 191 on the upstream of the compressor 120 andthe second valve 192 on a downstream of the cooler 130 are closed sothat the boil-off gas is supplied to the fuel consumption place 180 viathe first extra compressor 122 and the first extra cooler 132.

The present invention is to increase the re-liquefaction efficiency andre-liquefaction amount of the boil-off gas by using the first extracompressor 122 and the first extra cooler 132 which are not used even ifthey are installed in the ship, and sends a part of the boil-off gascompressed by the first extra compressor 122 to the fuel consumptionplace 180 and uses the other part of the boil-off gas as a refrigerantwhich additionally cools the boil-off gas in the refrigerant heatexchanger 140.

FIG. 7 is a graph schematically illustrating a phase change of methanedepending on temperature and pressure. Referring to FIG. 7, methanebecomes a supercritical fluid state at a temperature of approximately−80° C. or higher and a pressure of approximately 55 bars or higher.That is, in the case of methane, a critical point is approximately −80°C. and 55 bars. The supercritical fluid state is a third state differentfrom a liquid phase or a gas phase.

On the other hand, if the supercritical fluid states has a temperaturelower than the critical point at a pressure equal to or higher than thecritical point, it may also be a state in which a density is high,unlike a general liquid phase. Here, the state of the boil-off gashaving a pressure equal to or higher than the critical point and atemperature equal to lower than the critical point is referred to as a‘high-pressure liquid phase’.

The boil-off gas compressed by the compressor 120 or the first extracompressor 122 may be in a gaseous state or in a supercritical fluidstate depending on how much the boil-off gas is compressed.

When the boil-off gas sent to the boil-off gas heat exchanger 110through the return line L3 is in a gas phase, the temperature of theboil-off gas is lowered while the boil-off gas passes through theboil-off gas heat exchanger 110, and thus the boil-off gas may be amixed state of liquid and gas. In the case of the supercritical fluidstate, the temperature of the boil-off gas is lowered while the boil-offgas passes through the boil-off gas heat exchanger 110 and thus theboil-off gas may be the ‘high-pressure liquid phase’.

The temperature of the boil-off gas cooled by the boil-off gas heatexchanger 110 is further lowered while the boil-off gas passes throughthe refrigerant exchanger 140. When the boil-off gas passing through theboil-off gas heat exchanger 110 is in the mixed state of liquid and gas,the temperature of the boil-off gas is further lowered while theboil-off gas passes through the refrigerant heat exchanger 140 and thusthe boil-off gas becomes the mixed state in which a ratio of liquid ishigher or becomes the liquid phase and in the case of the ‘high-pressureliquid phase’, the temperature of the boil-off gas is further loweredwhile the boil-off gas passes through the refrigerant heat exchanger140.

Further, even when the boil-off gas which passes through the refrigerantheat exchanger 140 is in the ‘high-pressure liquid phase’, the pressureof the boil-off gas is lowered while the boil-off gas passes through thefirst decompressing device 150, and thus the boil-off gas becomes low ina liquid phase or the mixed state of liquid and gas.

It can be appreciated that even if the pressure of the boil-off gas islowered to the same level (P in FIG. 7) by the first decompressingdevice 150, the boil-off gas becomes the mixed state in which the rationof the liquid is higher in the case where the boil-off gas isdecompressed in the higher temperature (X→X′ in FIG. 7) than in the casewhere the boil-off gas is decompressed in the lower temperature (Y→Y′ inFIG. 7). Further, it can be appreciated that if the temperature may befurther lowered, the boil-off gas can theoretically be re-liquefied 100%(Z→Z′ in FIG. 7). Therefore, if the boil-off gas is cooled once more bythe refrigerant heat exchanger 140 before passing through the firstdecompressing device 150, the re-liquefaction efficiency and theliquefaction amount can be increased.

Referring back to FIG. 4, compared with the first and second embodimentsin which the refrigerant circulators 300 a and 300 b for additionallycooling the boil-off gas are configured as the closed loop, the presentembodiment is different from the first and second embodiments in thatthe refrigerant cycle is configured as the open loop.

In the first and second embodiments, the refrigerant circulators 300 aand 300 b are configured as the closed loop, and thus the boil-off gascompressed by the refrigerant compressors 310 a and 310 b is used onlyas a refrigerant in the refrigerant heat exchangers 500 a and 500 b butmay not be sent to the fuel consumption place or may not undergo there-liquefaction process.

On the other hand, in the present embodiment, the refrigerant cycle isconfigured as the open loop, and thus the boil-off gas compressed by thefirst extra compressor 122 is joined with the boil-off gas compressed bythe compressor 120, and then a part of the boil-off gas is sent to thefuel consumption place 180, the other part thereof is used as therefrigerant in the refrigerant heat exchanger 140 along therecirculation line L5, and the remaining part thereof undergoes there-liquefaction process along the return line L3.

The recirculation line L5 is a line which is branched from the firstsupply line L1 on the downstream of the compressor 120 and connected tothe first supply line L1 on the upstream of the compressor 120. A sixthvalve 196 which controls the flow rate of the boil-off gas and theopening/closing thereof may be installed on the recirculation line L5along which the boil-off gas branched from the first supply line L1 issent to the refrigerant heat exchanger 140.

Compared with the first and second embodiments in which the refrigerantcycle is configured as the closed loop, the present embodiment in whichthe refrigerant cycle is configured as the open loop is greatlydifferent from the first and second embodiments in that the downstreamline of the compressor 120 and the downstream line of the first extracompressor 122 are connected. That is, in the present embodiment, thesecond supply line L2 on the downstream of the first extra compressor122 is connected to the first supply line L1 on the downstream of thecompressor 120, and thus the boil-off gas compressed by the first extracompressor 122 is joined with the boil-off gas compressed by thecompressor 120 and then sent to the refrigerant heat exchanger 140, thefuel consumption place 180, or the boil-off gas heat exchanger 110. Thepresent embodiment includes all other modifications in which thedownstream of the compressor 120 and the downstream line of the firstextra compressor 122 are connected.

Therefore, according to the present embodiment, upon the increase in thedemanded amount of the fuel consumption place 180 such as the increasein the operating speed of the ship, the boil-off gas compressed by thefirst extra compressor 122 as well as the boil-off gas compressed by thecompressor 120 as well as the compressed may be sent to the fuelconsumption place 180.

Generally, however, since the compressor 120 and the first extracompressor 122 are designed to have a capacity of approximately 1.2times the amount required in the fuel consumption place 180, the case inwhich the boil-off gas compressed by the first extra compressor 122exceeding the capacity of the compressor 120 is sent to the fuelconsumption place 180 little occurs. Rather, since the boil-off gasdischarged from the storage tank T are entirely not consumed in the fuelconsumption place 180 and therefore the boil-off gas to be re-liquefiedincreases, the case in which a large amount of refrigerant is requiredto re-liquefy a large amount of boil-off gas is more frequent.

According to the present embodiment, since not only the boil-off gascompressed by the compressor 120 but also the boil-off gas compressed bythe first extra compressor 122 may be used as the refrigerant for theheat exchange in the refrigerant heat exchanger 140, the boil-off gassupplied to the refrigerant heat exchanger 140 along the return line L3after passing through the boil-off gas heat exchanger 110 may be cooledto a lower temperature by using more refrigerant and the overallre-liquefaction efficiency and re-liquefaction amount may be increased.Theoretically, 100% re-liquefaction is possible.

Generally, upon determining the capacity of the compressors 120 and 122provided in the ship, both of the capacity required for supplying theboil-off gas to the fuel consumption place 180 and the capacity requiredfor re-liquefying the boil-off gas remaining by being not completelyconsumed in the fuel consumption place 180 are considered. According tothe present embodiment, since the re-liquefaction amount may beincreased by using the extra compressor 122, the capacity required forre-liquefaction may be reduced, and thus small-capacity compressors 120and 122 can be provided. Reducing the capacity of the compressor cansave both equipment installation costs and operating costs.

In the present embodiment, at ordinary times when the compressor 120 orthe cooler 130 does not fail, not only the first valve 191 and thesecond valve 192 but also the third valve 193 and the fourth valve 194are open so that all of the compressor 120, the cooler 130, the firstextra compressor 122, and the first extra cooler 132 are operated, andwhen the compressor 120 or the cooler 130 fails, increasing there-liquefaction efficiency and the re-liquefaction amount is abandonedand the first valve 191 and the second valve 192 are closed so that thesystem is operated only by the boil-off gas passing through the firstextra compressor 122 and the first extra compressor 132.

For convenience of explanation, it is described that the compressor 120and the cooler 130 play a major role and the first extra compressor 122and the first extra cooler 132 play an auxiliary role. However, thecompressor 120 and the first extra compressor 122 and the cooler 130 andthe first extra cooler 132 play the same role. At least two compressorsand coolers which play the same role are installed in one ship, andtherefore when any one of the two compressors fail, the other unbrokencompressor may be used, which may satisfy a redundancy concept. Next,the above description is applied.

Therefore, as in the case in which the compressor 120 or the cooler 130fails, even in the case in which the first extra compressor 122 or thefirst extra cooler 132 fails, increasing the re-liquefaction efficiencyand the re-liquefaction amount is abandoned, and the third valve 193 andthe fourth valve 194 are closed so that the system is operated only theboil-off gas passing through the compressor 120 and the cooler 130.

On the other hand, when the ship is operated at a high speed enough thatmost or all of the boil-off gas discharged from the storage tank T canbe used as fuel for the fuel consumption place 180, there is little orno amount of boil-off gas. Accordingly, when the ship is operated at ahigh speed, only one of the compressor 120 and the first extracompressor 122 may be operated.

The compressor 120 and the first extra compressor 122 may compress theboil-off gas to a pressure required by the fuel consumption place 180.The fuel consumption place 180 may be an engine, a generator, or thelike which are operated by the boil-off gas as fuel. For example, if thefuel consumption place 180 is a boost engine for a ship, the compressor120 and the first extra compressor 122 may compress the boil-off gas toa pressure of approximately 10 to 100 bars.

In addition, the compressor 120 and the first extra compressor 122 mayalso compress the boil-off gas to a pressure of approximately 150 barsto 400 bars when the fuel consumption place 180 is an ME-GI engine, andwhen the fuel consumption place 180 is a DFDE, the boil-off gas may becompressed to a pressure of approximately 6.5 bars, and when the fuelconsumption place 180 is an X-DF engine, the boil-off gas may becompressed to a pressure of approximately 16 bars.

The fuel consumption place 180 may also include various kinds ofengines. For example, when the fuel consumption place 180 includes theX-DF engine and the DFDE, the compressor 120 and the first extracompressor 122 may compress the boil-off gas to the pressure required bythe X-DF engine, and the decompressing device is installed on theupstream of the DFDE to lower a part of the boil-off gas compressed atthe pressure required by the X-DF engine to a pressure required by theDFDE and then supply the compressed boil-off gas to the DFDE.

In addition, in order to increase the re-liquefaction efficiency and there-liquefaction amount in the boil-off gas heat exchanger 110 and therefrigerant heat exchanger 140, the compressor 120 or the first extracompressor 122 compresses the boil-off gas so that the pressure of theboil-off gas exceeds the pressure required by the fuel consumption place180, and the decompressing device is installed on the upstream of thefuel consumption place 180 to lower the pressure of the compressedboil-off gas to exceed the pressure required by the fuel consumptionplace 180 to the pressure required by the fuel consumption place 180 andthen supply the compressed boil-off gas to the fuel consumption place180.

Meanwhile, the compressor 120 and the first extra compressor 122 mayeach be a multi-stage compressor. FIG. 4 illustrates that one compressor120 or 122 compresses the boil-off gas to the pressure required by thefuel consumption place 180, but when the compressor 120 and the firstextra compressor 122 are a multi-stage compressor, a plurality ofcompression cylinders may compress the boil-off gas to the pressurerequired by the fuel consumption place 180 several times.

When the compressor 120 and the first extra compressor 122 are amulti-stage compressor, the plurality of compression cylinders may beprovided in the compressor 120 and the first extra compressor 122 inseries and the plurality of coolers may each be provide on thedownstream of the plurality of compression cylinders.

The cooler 130 of the present embodiment is installed downstream of thecompressor 120 to cool the boil-off gas which is compressed by thecompressor 120 and has the increased pressure and temperature. The firstextra cooler 132 of the present embodiment is installed downstream ofthe first extra compressor 122 to cool the boil-off gas which iscompressed by the first extra compressor 122 and has the increasedpressure and temperature. The cooler 130 and the first extra cooler 132may cool the boil-off gas by exchanging heat with seawater, fresh water,or air introduced from the outside.

The refrigerant heat exchanger 140 of the present embodimentadditionally cools the boil-off gas which is cooled by the boil-off gasheat exchanger 110 and then supplied to the refrigerant heat exchanger140 along the return line L3. The refrigerant decompressing device 160of the present embodiment expands the boil-off gas which passes throughthe refrigerant heat exchanger 140 and then sends the expanded boil-offgas back to the refrigerant heat exchanger 140.

That is, the refrigerant heat exchanger 140 expands the boil-off gas,which passes through the boil-off gas heat exchanger 110 and thensupplied to the refrigerant heat exchanger 140 along the return line L3,performs heat exchange by the refrigerant to additionally cool theboil-off gas expanded by the refrigerant decompressing device 160.

The refrigerant decompressing device 160 of the present embodiment maybe various means for lowering the pressure of the fluid, and the stateof the fluid just before passing through the refrigerant decompressingdevice 160 and the state of the fluid just after passing through therefrigerant decompressing device 160 may be changed depending on theoperation condition of the system. However, when the refrigerantdecompressing device 160 is an expander, in order to prevent a physicaldamage of the refrigerant decompressing device 160, the fluid justbefore passing through the refrigerant decompressing device 160 and thefluid just after passing through the refrigerant decompressing device160 is preferably maintained in a gas phase. Next, the above descriptionis applied.

By the boil-off gas used as the refrigerant for the heat exchange in therefrigerant heat exchanger 140 after passing through the refrigerantdecompressing device 160, after the boil-off gas compressed by thecompressor 120 is joined with the boil-off gas compressed by the firstextra compressor 122, a part of the joined boil-off gas is supplied tothe refrigerant heat exchanger 140 along the recirculation line L5 andcooled by exchanging heat with the boil-off gas, which passes throughthe refrigerant decompressing device 160, in the refrigerant heatexchanger 140 by the refrigerant and then supplied to the refrigerantdecompressing device 160.

In addition, the boil-off gas supplied from the first supply line L1 tothe refrigerant heat exchanger 140 along the first supply line L1 isprimarily used in the refrigerant heat exchanger 140 and is additionallycooled by the refrigerant decompressing device 160 and is then sent backto the refrigerant heat exchanger 140, such that the boil-off gas isused as the refrigerant.

That is, the flow of the boil-off gas compressed by the compressor 120supplied to the refrigerant heat exchanger 140 along the recirculationline L5 after being joined with the boil-off gas compressed by the firstextra compressor 122 and the boil-off gas which passes through theboil-off gas heat exchanger 110 and is then supplied to the refrigerantheat exchanger 140 along the return line L3 exchange heat with eachother by the boil-off gas, which passes through the refrigerantdecompressing device 160, as a refrigerant to be cooled.

The first decompressing device 150 of the present embodiment isinstalled on the return line L3 to expand the boil-off gas cooled by theboil-off gas heat exchanger 110 and the refrigerant heat exchanger 140.The boil-off gas compressed by the compressor 120 is joined with theboil-off gas compressed by the first extra compressor 122 and then apart of the boil-off gas is branched into pass through the boil-off gasheat exchanger 110, the refrigerant heat exchanger 110 140, and thefirst decompressing device 150, such that the boil-off gas is partiallyor totally re-liquefied.

The first decompressing device 150 includes all means which may expandand cool the boil-off gas, and may be an expansion valve, such as aJoule-Thomson valve, or an expander.

The ship of the present embodiment may include the gas-liquid separator170 which is installed on the return line L3 on the downstream of thefirst decompressing device 150 and separates the gas-liquid mixturedischarged from the first decompressing device 150 into gas and liquid.

When the ship of the present embodiment does not include the gas-liquidseparator 170, the liquid or the boil-off gas in the gas-liquid mixedstate which passes through the first decompressing device 150 isimmediately sent to the storage tank T.

When the ship of the present embodiment includes the gas-liquidseparator 170, the boil-off gas which passes through the firstdecompressing device 150 is sent to the gas-liquid separator 170 toseparate the gas phase and the liquid phase. The liquid separated by thegas-liquid separator 170 returns to the storage tank T along the returnline L3 and the gas separated by the gas-liquid separator 170 issupplied to the boil-off gas heat exchanger 110 along a gas dischargeline which extends from the gas-liquid separator 170 to the first supplyline L1 on the upstream of the boil-off gas heat exchanger 110.

When the ship of the present embodiment includes the gas-liquidseparator 170, the ship may further include a seventh valve 197 whichcontrols the flow rate of the liquid separated by the gas-liquidseparator 170 and sent to the storage tank T; an eighth valve 198 whichcontrols the flow rate of gas separated by the gas-liquid separator 170and sent to the boil-off gas heat exchanger 110.

The first to eighth valves and the eleventh valve 191, 192, 193, 194,195, 196, 197, 198, and 203 of the present embodiment may be manuallycontrolled by allowing a person to directly determine the operationcondition of the system and may be automatically controlled to be openedor closed depending on a preset value.

The main flow of the boil-off gas is defined to easily describe theoperation of the device for re-liquefaction of boil-off gas according toan embodiment of the present invention. A flow in which the boil-off gasgenerated from the storage tank T and the gas discharged from thegas-liquid separator 170 is supplied to the boil-off gas heat exchanger110 is defined as a first flow 100, a flow which is supplied from theboil-off gas heat exchanger 110 to the compressor 120 and the firstextra compressor 122 and then discharged from the compressor 120 or thefirst extra compressor 122 and supplied to the fuel consumption place180 is defined as a second flow 102, a flow which is branched from thesecond flow 102 on the downstream of the compressor 120 and the firstextra compressor 122 and the supplied to the refrigerant heat exchanger140 is defined as a third flow 104, a flow which is branched from thesecond flow 102 on the downstream of the compressor 120 and the firstextra compressor 122 and supplied to the boil-off gas heat exchanger 110is defined as a fourth flow 106, and a flow which is supplied from theboil-off gas heat exchanger 110 to the refrigerant heat exchanger 140 isdefined as a fifth flow 108. The first flow 100 becomes the second flowwhile passing through the boil-off heat exchanger 110 and the fourthflow 106 becomes the fifth flow 108 while passing through the boil-offheat exchanger 110.

Hereinafter, an operation of an apparatus for re-liquefaction ofboil-off gas re-according to an embodiment of the present invention willbe described with reference to FIG. 4. The present embodiment isparticularly suitable for the case where the liquefied gas stored in thestorage tank is liquefied natural gas and the fuel consumption place isX-DF, but is not limited thereto. The same goes for the fourthembodiment.

The gaseous boil-off gas generated from the storage tank (T) storing theliquefied gas in the liquid phase is supplied to the boil-off gas heatexchanger (110). At this time, the gaseous boil-off gas generated fromthe storage tank T meets the gaseous boil-off gas discharged from thegas-liquid separator 170 after the predetermined time elapses from theoperation of the system to form the first flow 100. Ultimately, theboil-off gas supplied to the boil-off gas heat exchanger 110 becomes thefirst flow 100.

The boil-off gas heat exchanger 110 serves to recover the cold heat ofthe first flow 100 to cool the other boil-off gas. That is, the boil-offheat exchanger 110 recovers the cold heat of the first flow 100 anddelvers the recovered cold heat to the flow supplied back to theboil-off heat exchanger 110 in the second flow 102, that is, the fourthflow 106.

Accordingly, in the boil-off heat exchanger 110, the heat exchange isgenerated between the first flow 100 and the fourth flow 106 so that thefirst flow 100 is heated and the fourth flow 106 is cooled. The heatedfirst flow 100 becomes the second flow 102 and the cooled fourth flow106 becomes the fifth flow 108.

The second flow 102 discharged from the boil-off gas heat exchanger 110is supplied to the compressor 120 or the first extra compressor 122 andis compressed by the compressor 120 or the first extra compressor 122.

A part of the second flow 102 in which the boil-off gas compressed bythe compressor 120 and the boil-off gas compressed by the first extracompressor 122 are joined is the third flow 104 and supplied to therefrigerant heat exchanger 140 as a refrigerant, and the other partthereof is the fourth flow 106 and is supplied to the boil-off gas heatexchanger 110 to be cooled and the remaining part thereof is supplied tothe fuel consumption place 180.

The third flow 104 supplied to the refrigerant heat exchanger 140 isdischarged from the refrigerant heat exchanger 140 and expanded in therefrigerant decompressing device 160 and then supplied back to therefrigerant heat exchanger 140. At this time, the third flow 104primarily supplied to the refrigerant heat exchanger 140 is expanded inthe refrigerant heat exchanger 140 and then exchanges heat with thethird flow 104 supplied back to the refrigerant heat exchanger 140 to becooled. The third flow 104 which passes through the refrigerantdecompressing device 160 and the refrigerant heat exchanger 140 isjoined with the second flow 102 that is discharged from the boil-off gasheat exchanger 110 and supplied to the compressor 120 or the first extracompressor 122.

The fourth flow 106 cooled by the heat exchange with the first flow 100in the boil-off gas heat exchanger 110 becomes the fifth flow 108 and issupplied to the refrigerant heat exchanger 140. The fifth flow 108supplied to the refrigerant heat exchanger 140 exchanges heat with thethird flow 104 which passes through the refrigerant decompressing device160 and is cooled, and then passes through the first decompressingdevice 150 and expanded. The fifth flow 108 which passes through thefirst decompressing device 150 becomes a gas-liquid mixture state inwhich gas and liquid are mixed.

The fifth flow 108 in the gas-liquid mixture state is immediately sentto the storage tank T or separated into gas and liquid while passingthrough the gas-liquid separator 170. The liquid separated by thegas-liquid separator 170 is supplied to the storage tank T and the gasseparated by the gas-liquid separator 170 is supplied back to theboil-off gas heat exchanger 110, thereby repeating the above-mentionedseries of processes.

FIG. 5 is a configuration diagram schematically showing a boil-off gastreatment system for a ship according to a fourth embodiment of thepresent invention.

The ship of the fourth embodiment shown in FIG. 5 is different from theship of the third embodiment shown in FIG. 4 in that the ship of thefourth embodiment further includes a third supply line L7 along which apart of the boil-off gas passing through the boil-off gas heat exchanger112 along the return line L3 is branched to be sent back to the boil-offgas heat exchanger 112 to increase the re-liquefaction efficiency andthe re-liquefaction amount in the boil-off gas heat exchanger 112,further includes a ninth valve 201, a first additional line L6, a tenthvalve 202, and a thirteenth valve 205, and a system is configured tomodify some lines in which the boil-off gas flows to operate therefrigerant cycle as the closed loop as in the first embodiment and thesecond embodiment and operate the refrigerant cycle as the closed loopas in the third embodiment. Hereinafter the difference will be mostlydescribed. The detailed description of the same member as the ship ofthe foregoing third embodiment will be omitted.

In addition, the fuel consumption places 180 and 181 of the presentembodiment are configured to correspond to the fuel consumption place180 of the fourth embodiment. However, to differentiate the fuelconsumption place 180 using the high-pressure boil-off gas as fuel fromthe fuel consumption place 181 using the low-pressure boil-off gas asfuel, the fuel consumption place using the high-pressure boil-off gas asfuel is referred to as a high pressure engine 180 and the fuelconsumption place using the low-pressure boil-off gas is referred to alow-pressure engine 181.

The ‘high pressure’ of the high pressure engine 180 means that it uses ahigher pressure fuel than the low pressure engine 181 and may be an X-DFengine as well as an ME-GI engine, which is generally classified as ahigh pressure engine. The low pressure engine 181 may be a DF engine ora gas turbine and the low pressure engine 181 may be the X-DF engine ifthe high pressure engine 180 is an ME-GI engine. Next, the abovedescription is applied.

Referring to FIG. 5, similar to the third embodiment, the ship of thepresent embodiment includes the boil-off gas heat exchanger 112, thefirst valve 191, the compressor 120, the cooler 130, the second valve192, the third valve 193, the extra compressor 122, the extra cooler132, the fourth valve 194, the refrigerant heat exchanger 140, therefrigerant decompressing device 160, and the first decompressing device150.

Similar to the third embodiment, the storage tank T stores liquefied gassuch as liquefied natural gas and liquefied ethane gas, and dischargesthe boil-off gas to the outside when the internal pressure of thestorage tank T exceeds a certain pressure or higher. The boil-off gasdischarged from the storage tank (T) is sent to the boil-off gas heatexchanger 112.

Similar to the third embodiment, the boil-off gas heat exchanger 112 ofthe present embodiment uses the boil-off gas discharged from the storagetank T as the refrigerant and cools the boil-off gas supplied to theboil-off gas heat exchanger 112 along the return line L3.

However, the boil-off gas heat exchanger 112 of the present embodimentis different from the third embodiment in that the boil-off gas heatexchanger 112 uses as a refrigerant, the boil-off gas which passesthrough the boil-off gas heat exchanger 112 along the return line L3 andis then supplied along the third supply line L7 branched from the returnline L3 as well as the boil-off gas discharged from the storage tank Talong the return line L3 Therefore, the boil-off gas heat exchanger 112of the present embodiment is a multiple heat exchanger.

The boil-off gas heat exchanger 112 recovers cold heat of the boil-offgas discharged from the storage tank T and cold heat of the boil-off gassupplied along the third supply line L7 and supplies the recovered coldheat to the boil-off gas sent to the boil-off gas heat exchanger 112along the return line L3.

According to this embodiment, since not only the boil-off gas dischargedfrom the storage tank T but also the boil-off gas supplied along thethird supply line L7 is used as the refrigerant, the re-liquefactionefficiency and the re-liquefaction amount can more be increased than thethird embodiment.

The present embodiment is particularly meaningful when the ship includesa plurality of engines having different fuel pressures required.

When the fuel pressure required by a plurality of engines mounted on theship is different from each other, the boil-off gas discharged from thestorage tank T is compressed with the pressure required by the highpressure engine 180 by the compressor 120 or the extra compressor 122and a part of the boil-off gas compressed with the pressure required bythe high pressure engine 180 is branched and decompressed and then sentto the low pressure engine 181. At this time, to send the fuel to thelow pressure engine 181, the temperature of the boil-off gas is loweredwhile the boil-off gas is decompressed.

Accordingly, according to the present embodiment, the decompressed andcooled boil-off gas is not directly sent to the low pressure engine 181but is supplied to the low pressure engine 181 via the boil-off gas heatexchanger 112, and the boil-off gas heat exchanger 112 may use the coldheat of the decompressed and cooled boil-off gas.

However, it should be noted that the compressor 120 or the extracompressor 122 of the present embodiment does not necessarily compressthe boil-off gas only up to the pressure required by the high-pressureengine 180 but may compress the boil-off gas to a pressure higher thanthe pressure required by the high pressure engine 180 to increase theheat exchange efficiency in the heat exchanger 112 and the refrigerantheat exchanger 140. The boil-off gas compressed to a pressure higherthan the pressure required by the high-pressure engine 180 may besupplied to the high pressure engine 180 after being decompressed up tothe pressure required by the high-pressure engine 180 at the front endof the high pressure engine 180

A fifth valve 195 which controls the flow rate of the boil-off gas andthe opening and closing may be installed on the return line L3 and atwelfth valve 204 which controls the flow rate of the boil-off gas andthe opening and closing may be installed on the third supply line L7 onthe upstream of the low pressure engine 181.

Similar to the third embodiment, the compressor 120 of the presentembodiment is installed on the first supply line L1 to compress theboil-off gas discharged from the storage tank T and similar to the thirdembodiment, the extra compressor 122 of the present embodiment isinstalled in parallel with the compressor 120 on the second supply lineL2 to compress the boil-off gas discharged from the storage tank T. Thecompressor 120 and the extra compressor 122 may be a compressor havingthe same performance, and each may be a multi-stage compressor.

Similar to the third embodiment, the ship of the present embodiment mayfurther include an eleventh valve 203 which is installed on the upstreamof the high-pressure engine 180 to control a flow rate of the boil-offgas sent to the high pressure engine 180 and the opening/closingthereof.

Similar to the third embodiment, the ship of the present embodiment usesthe boil-off gas compressed by the extra compressor 122 as therefrigerant which additionally cools the boil-off gas in the refrigerantheat exchanger 140, thereby increasing the re-liquefaction efficiencyand the re-liquefaction amount.

Similar to the third embodiment, the cooler 130 of the presentembodiment is installed downstream of the compressor 120 to cool theboil-off gas that passes through the compressor 120 and has theincreased pressure and temperature. Similar to the third embodiment, theextra cooler 132 of the present embodiment is installed downstream ofthe extra compressor 122 to cool the boil-off gas which passes throughthe extra compressor 122 and has the increased pressure and temperature.

Similar to the third embodiment, the refrigerant heat exchanger 140 ofthe present embodiment additionally cools the boil-off gas which issupplied to the boil-off gas heat exchanger 112 along the return line L3and cooled by the boil-off gas heat exchanger 112.

Similar to the third embodiment, according to the present embodiment,the boil-off gas discharged from the storage tank T is further coolednot only in the boil-off gas heat exchanger 112 but also in therefrigerant heat exchanger 140, and may be supplied to the firstdecompressing device 150 in the state in which the temperature is lower,thereby increasing the re-liquefaction efficiency and there-liquefaction amount. In addition, as described above, the boil-offgas is cooled by the cold heat of the boil-off gas sent from theboil-off gas heat exchanger 112 to the low pressure engine 181, therebymore increasing the re-liquefaction efficiency and the re-liquefactionamount than the third embodiment.

Similar to the third embodiment, the refrigerant decompressing device160 of the present embodiment expands the boil-off gas which passesthrough the refrigerant heat exchanger 140, and then sends the expandedboil-off gas back to the refrigerant heat exchanger 140.

Similar to the third embodiment, the first decompressing device 150 ofthe present embodiment is installed on the return line L3 to expand theboil-off gas cooled by the boil-off gas heat exchanger 112 and therefrigerant heat exchanger 140. The first decompressing device 150 ofthe present embodiment includes all means which may expand and cool theboil-off gas, and may be an expansion valve, such as a Joule-Thomsonvalve, or an expander.

Similar to the third embodiment, the ship of the present embodiment mayinclude the gas-liquid separator 170 which is installed on the returnline L3 on the downstream of the first decompressing device 150 andseparates the gas-liquid mixture discharged from the first decompressingdevice 150 into gas and liquid.

Similar to the third embodiment, when the ship of the present embodimentdoes not include the gas-liquid separator 170, the liquid or theboil-off gas in the gas-liquid mixed state which passes through thefirst decompressing device 150 is directly sent to the storage tank T,and when the ship of the present embodiment includes the gas-liquidseparator 170, the boil-off gas which passes through the firstdecompressing device 150 is sent to the gas-liquid separator 170 to beseparated into the gas phase and the liquid phase. The liquid separatedby the gas-liquid separator 170 returns to the storage tank T along thereturn line L3 and the gas separated by the gas-liquid separator 170 issupplied to the boil-off gas heat exchanger 112 along a gas dischargeline which extends from the gas-liquid separator 170 to the first supplyline L1 on the upstream of the boil-off gas heat exchanger 112.

Similar to the third embodiment, when the ship of the present embodimentincludes the gas-liquid separator 170, the ship may further include aseventh valve 197 which controls the flow rate of the liquid separatedby the gas-liquid separator 170 and sent to the storage tank T; aneighth valve 198 which controls the flow rate of gas separated by thegas-liquid separator 170 and sent to the boil-off gas heat exchanger112.

However, unlike the third embodiment, the ship of the present embodimentfurther includes: a third supply line L7 on which a part of the boil-offgas passing through the boil-off gas heat exchanger 112 along the returnline L3 is branched and sent back to the boil-off gas heat exchanger112; a third decompressing device 301 which is installed on the thirdsupply line L7 to decompress the boil-off gas sent to the boil-off gasheat exchanger 112; a first additional line L6 which connects betweenthe recirculation line L5 and the second supply line L2; a ninth valve201 which is installed on the recirculation line L5; a tenth valve 202which is installed on the first additional line L6; and a thirteenthvalve 205 which is installed on the recirculation line L5 between thesecond supply line L2 and the refrigerant heat exchanger 140.

Unlike the third embodiment which selectively includes the sixth valve196, the ship of the present embodiment essentially includes the sixthvalve 196 which is installed on the recirculation line L5 along whichthe boil-off gas branched from the first supply line L1 is sent to therefrigerant heat exchanger 140 to control the flow rate of the boil-offgas and the opening/closing thereof.

The third supply line L7 of the present embodiment is branched from thereturn line L3 between the boil-off gas heat exchanger 112 and therefrigerant heat exchanger 140 and a part of the boil-off gas which iscompressed by the compressor 120 or the extra compressor 122 and thenpasses through the boil-off gas heat exchanger 112 along the return lineL3 is sent back to the boil-off gas heat exchanger 112. The boil-off gassent to the boil-off gas heat exchanger 112 along the third supply lineL7 is used as a refrigerant in the boil-off gas heat exchanger 112.

The third decompressing device 301 of the present embodiment isinstalled on the third supply line L7 on the upstream of the boil-offgas heat exchanger 112 and expands the partially branched boil-off gasof the boil-off gas which is cooled by the boil-off gas heat exchanger112 and then returns to the refrigerant heat exchanger 140 along thereturn line L3. The boil-off gas expanded by the third decompressingdevice 301 has the lowered pressure and temperature. The boil-off gaswhose temperature is lowered by the third decompressing device 301supplies the cold heat to the boil-off gas heat exchanger 112.

The third decompressing device 301 of the present embodiment includesall means which may expand and cool the boil-off gas, and may be anexpansion valve, such as a Joule-Thomson valve, or an expander.

One side of the first additional line L6 of the present embodiment isconnected to a recirculation line (not shown) which is expanded by therefrigerant decompressing device 160 and then sent to the first supplyline L1 through the refrigerant heat exchanger 140 L5 and the other sidethereof is connected to the second supply line L2 between the thirdvalve 193 and the extra compressor 122.

The ninth valve 201 of the present embodiment is installed on therecirculation line L5 between the point where the recirculation line L5meets the first supply line L1 on the upstream of the compressor 120 andthe extra compressor 122 and the point where the recirculation line L5meets the first additional line L6, thereby controlling the flow rate ofthe fluid and the opening/closing thereof.

In addition, the ship of the present embodiment is different from thethird embodiment in that the second supply line L2 on the downstream ofthe extra compressor 122 is connected to the recirculation line L5instead of the first supply line L1.

The first to thirteenth valves and the eleventh valve 191, 192, 193,194, 195, 196, 197, 198, 201, 202, 203, 204, and 205 of the presentembodiment may be manually controlled by allowing a person to directlydetermine the operation condition of the system and may be automaticallycontrolled to be opened or closed depending on a preset value.

Unlike the third embodiment, according to the ship of the presentembodiment, the refrigerant cycle may be operated not only as the openloop but also as the closed loop so as to more flexibly use there-liquefaction system according to the operating conditions of theship. Hereinafter, a method of operating a refrigerant cycle as theclosed loop and a method of operating a refrigerant cycle as the openloop by a valve control will be described.

To operate the refrigerant cycle of the ship of the present embodimentas the closed loop, the system is operated while the first valve 191,the second valve 192, the third valve 193, the fourth valve 194, thetenth valve 202, and the thirteenth valve 205 are open, and the sixthvalve 196 and the ninth valve 201 are closed.

If the boil-off gas which is discharged from the storage tank T and thencompressed by the extra compressor 122 is supplied to the recirculationline L5, the third valve 193 is closed to form the refrigerant cycle ofthe closed loop in which the boil-off gas circulates the extracompressor 122, the extra cooler 132, the fourth valve 194, thethirteenth valve 205, the refrigerant heat exchanger 140, therefrigerant decompressing device 160, the refrigerant heat exchanger140, and the tenth valve 202.

When the refrigerant cycle is configured as the closed loop, nitrogengas may be used as the refrigerant circulating the closed loop. In thiscase, the ship of the present embodiment may further include the pipethrough which the nitrogen gas is introduced into the refrigerant cycleof the closed loop.

When the refrigerant cycle is operated as the closed loop, only theboil-off gas circulating the closed loop is used as the refrigerant inthe refrigerant heat exchanger 140. The boil-off gas passing through thecompressor 120 is not introduced into the refrigerant cycle but issupplied to the high-pressure engine 180 or undergoes there-liquefaction process along the return line L3. Therefore, apredetermined flow rate of boil-off gas is circulated as the refrigerantin the refrigerant heat exchanger 140 irrespective of there-liquefaction amount or the amount of boil-off gas required by thehigh pressure engine 180.

The flow of the boil-off gas in the case where the refrigerant cycle ofthe ship of the present embodiment is operated as the closed loop willbe described as follows.

The boil-off gas discharged from the storage tank T passes through theboil-off gas heat exchanger 112 and then compressed by the compressor120, and a part thereof is cooled by the cooler 130 and then sent to thefuel consumption place 180, and the remaining part thereof is sent tothe boil-off gas heat exchanger 112 along the return line L3. Theboil-off gas sent to the boil-off gas heat exchanger 112 along thereturn line L3 exchanges heat with the boil-off gas discharged from thestorage tank T and the boil-off gas supplied to the boil-off gas heatexchanger 112 through the third supply line L7 to be cooled, and then isbranched into two flows.

One flow of the boil-off gases, which are cooled by the boil-off gasheat exchanger 112 and then branched into two flows, is continuouslysupplied to the refrigerant heat exchanger 140 along the return line L3and heat-exchanged in the refrigerant heat exchanger 140 to beadditionally cooled. The boil-off gas cooled by the boil-off gas heatexchanger 112 and the refrigerant heat exchanger 140 is expanded by thefirst decompressing device 150 and thus the boil-off gas is partially ortotally re-liquefied.

When the ship of the present embodiment does not include the gas-liquidseparator 170, the boil-off gas partially or totally re-liquefied isdirectly sent to the storage tank T. When the ship of the presentembodiment includes the gas-liquid separator 170, the boil-off gaspartially or totally re-liquefied is sent to the gas-liquid separator170. The gas separated by the gas-liquid separator 170 is joined withthe boil-off gas discharged from the storage tank T and sent to theboil-off gas heat exchanger 112. The liquid separated by the gas-liquidseparator 170 is supplied to the storage tank T.

The other flow of the boil-off gases, which are cooled by the boil-offgas heat exchanger 112 and then branched into two flows, is expanded bythe third decompressing device 301 along the third supply line L7 andthen expanded back to the boil-off gas heat exchanger 112. The boil-offgas, which is expanded by the third decompressing device 301 and has thelowered pressure and temperature, is used as the refrigerant in theboil-off gas heat exchanger 112. The boil-off gas used as therefrigerant in the boil-off gas heat exchanger 112 after passing throughthe third decompressing device 301 is supplied to the low pressureengine 181 along the third supply line L7.

Meanwhile, the boil-off gas circulating the refrigerant cycle iscompressed by the extra compressor 122, cooled by the extra cooler 132,and then sent to the refrigerant heat exchanger 140 along therecirculation line L5. The boil-off gas which passes through the extracompressor 122 and the extra cooler 132 and then sent to the refrigerantheat exchanger 140 is primarily heat-exchanged in the refrigerant heatexchanger 140 to be cooled and then sent to the refrigerantdecompressing device 160 to be secondarily expanded and cooled. Theboil-off gas which passes through the refrigerant decompressing device160 is sent back to the refrigerant heat exchanger 140 to be used as arefrigerant which cools the boil-off gas passing through the boil-offgas heat exchanger 112 and then supplied to the refrigerant heatexchanger 140 along the return line L3 and the boil-off gas compressedby the extra compressor 122 and then supplied to the refrigerant heatexchanger 140 along the recirculation line L5. The boil-off gas whichpasses through the refrigerant decompressing device 160 and then used asthe refrigerant in the refrigerant heat exchanger 140 is sent back tothe extra compressor 122, thereby repeating the above-mentioned seriesof processes.

When the compressor 120 or the cooler 130 fails while the refrigerantcycle of the ship of the present embodiment is operated as the closedloop, the first valve 191, the second valve 192, and the tenth valve202, and the thirteenth valve 205 are closed and the third valve 193 andthe sixth valve 196 are open to allow the boil-off gas which isdischarged from the storage tank T and then passes through the boil-offgas heat exchanger 112 to be supplied to the high pressure engine 180via the third valve 193, the extra compressor 122, the extra cooler 132,the fourth valve 194, and the sixth valve 196. When it is necessary touse the boil-off gas compressed by the extra compressor 122 as therefrigerant of the refrigerant heat exchanger 140, the ninth valve 201and the thirteenth valve 205 may be open to operate the system.

To operate the refrigerant cycle of the ship of the present embodimentas the open loop, the first valve 191, the second valve 192, the thirdvalve 193, the fourth valve 194, the sixth valve 196, the ninth valve201, and the thirteenth valve 205 are open, and the tenth valve 202 areclosed.

When the refrigerant cycle is operated as the closed loop, the boil-offgas circulating the refrigerant cycle and the boil-off gas sent to thehigh pressure engine 180 or undergoing the re-liquefaction process alongthe return line L3 are separated. On the other hand, when therefrigerant cycle is operated as the open loop, the boil-off gascompressed by the compressor 120 and the boil-off gas compressed by theextra compressor 122 are joined to be used as a refrigerant in therefrigerant heat exchanger 140, to be sent to the high pressure engine180, or to undergo the re-liquefying process along the return line L3.

Therefore, if the refrigerant cycle is operated as the open loop, theflow rate of the refrigerant to be sent to the refrigerant heatexchanger 140 may be flexibly controlled in consideration of there-liquefaction amount and the amount of boil-off gas required by thehigh pressure engine 180. In particular, when the amount of boil-off gasrequired by the high pressure engine 180 is small, increasing the flowrate of the refrigerant sent to the refrigerant heat exchanger 140 mayincrease the re-liquefaction efficiency and the re-liquefaction amount.

That is, when the refrigerant cycle is operated as the closed loop, itis not possible to supply the refrigerant heat exchanger 140 with theboil-off gas equal to or more than the capacity of the extra compressor122. However, when the refrigerant cycle is operated as the open loop,the boil-off gas having a flow rate exceeding the capacity of the extracompressor 122 may be supplied to the refrigerant heat exchanger 140.

The flow of the boil-off gas in the case where the refrigerant cycle ofthe ship of the present embodiment is operated as the open loop will bedescribed as follows.

The boil-off gas discharged from the storage tank T is branched into twoflows after passing through the boil-off gas heat exchanger 112 and apart thereof is sent to the first supply line L1 and the remaining partthereof is supplied to the second supply line L2.

The boil-off gas sent to the first supply line L1 passes through thefirst valve 191, the compressor 120, the cooler 130, and the secondvalve 192 and then a part thereof passes through the sixth valve 196 andthe thirteenth valve 205 and is sent to the refrigerant heat exchanger140, and the other part thereof is again branched into two flows. Oneflow of the boil-off gases branched into the two flows is sent to thehigh pressure engine 180 and the other thereof is sent to the boil-offgas heat exchanger 112 along the return line L3.

The boil-off gas sent to the second supply line L1 passes through thethird valve 193, the extra compressor 122, the extra cooler 132, and thefourth valve 194 and then a part thereof is sent to the refrigerant heatexchanger 140 via the thirteenth valve 205 and the other part thereof issent to the first supply line L1 and then branched into two flows. Oneflow of the boil-off gases branched into the two flows is sent to thehigh pressure engine 180 and the other flow thereof is sent to theboil-off gas heat exchanger 112 along the return line L3.

For convenience of explanation, the boil-off gas compressed by thecompressor 120 and the boil-off gas compressed by the extra compressor122 are separately described. However, each of the boil-off gascompressed by the compressor 120 and the boil-off gas compressed by theextra compressor 122 does not flow separately but is joined to besupplied to the refrigerant heat exchanger 140, the high pressure engine180, or the boil-off gas heat exchanger 112.

That is, the boil-off gas compressed by the compressor 120 and theboil-off gas compressed by the extra compressor 122 are mixed, which inturn flows in the recirculation line L5 along which the boil-off gas issent to the refrigerant heat exchanger 140, the first supply line L1along which the boil-off gas is sent to the high pressure engine 180,and the return line along which the boil-off gas is sent to the boil-offgas heat exchanger 112.

The boil-off gas sent to the refrigerant heat exchanger 140 along therecirculation line L5 is primarily heat-exchanged in the refrigerantheat exchanger 140 to be cooled, and secondarily expanded by therefrigerant decompressing device 160 to be cooled and supplied back tothe refrigerant heat exchanger 140. The boil-off gas which passesthrough the refrigerant decompressing device 160 and is then supplied tothe refrigerant heat exchanger 140 is used as the refrigerant whichcools both of the boil-off gas passing through the boil-off gas heatexchanger 112 and then supplied to the refrigerant heat exchanger 140along the return line L3 and the confluent flow of the boil-off gascompressed by the compressor 120 and the boil-off gas compressed by theextra compressor 122 which are supplied to the refrigerant heatexchanger 140 along the recirculation line L5.

That is, the boil-off gas used as the refrigerant in the refrigerantheat exchanger 140 is supplied to the refrigerant heat exchanger 140along the recirculation line L5, and then primarily cooled by therefrigerant heat exchanger 140 and secondarily cooled by the refrigerantdecompressing device 160. In addition, the boil-off gas sent from thecompressor 120 or the extra compressor 122 to the refrigerant heatexchanger 140 along the recirculation line L5 is primarily cooled by theboil-off gas, which passes through the refrigerant decompressing device160, as the refrigerant.

The boil-off gas used as the refrigerant in the refrigerant heatexchanger 140 after passing through the refrigerant decompressing device160 is sent to the first supply line L1 through the ninth valve 201 tobe discharged from the storage tank T and then joins with the boil-offgas passing through the boil-off gas heat exchanger 112, therebyrepeating the above-mentioned series of processes.

Meanwhile, the boil-off gas sent to the heat exchanger 112 along thereturn line L3 is cooled by the boil-off gas heat exchanger 112 and thenbranched into two flows.

One flow of the boil-off gases, which are cooled by the boil-off gasheat exchanger 112 and then branched into two flows, is continuouslysupplied to the refrigerant heat exchanger 140 along the return line L3and heat-exchanged in the refrigerant heat exchanger 140 to beadditionally cooled. The boil-off gas cooled by the boil-off gas heatexchanger 112 and the refrigerant heat exchanger 140 is expanded by thefirst decompressing device 150 and thus the boil-off gas is partially ortotally re-liquefied.

When the ship of the present embodiment does not include the gas-liquidseparator 170, the boil-off gas partially or totally re-liquefied isdirectly sent to the storage tank T. When the ship of the presentembodiment includes the gas-liquid separator 170, the boil-off gaspartially or totally re-liquefied is sent to the gas-liquid separator170. The gas separated by the gas-liquid separator 170 is joined withthe boil-off gas discharged from the storage tank T and sent to theboil-off gas heat exchanger 112. The liquid separated by the gas-liquidseparator 170 is supplied to the storage tank T.

The other stream of the boil-off gases, which are cooled by the boil-offgas heat exchanger 112 and then branched into two flows, is expanded bythe third decompressing device 301 along the third supply line L7 andthen expanded back to the boil-off gas heat exchanger 112. The boil-offgas, which is expanded by the third decompressing device 301 and has thelowered pressure and temperature, is used as the refrigerant in theboil-off gas heat exchanger 112. The boil-off gas used as therefrigerant in the boil-off gas heat exchanger 112 after passing throughthe third decompressing device 301 is supplied to the low pressureengine 181 along the third supply line L7.

When the compressor 120 or the cooler 130 fails while the refrigerantcycle of the ship of the present embodiment is operated as the openloop, the first valve 191, the second valve 192, the ninth valve 201,and the thirteenth valve 205 are closed to allow the boil-off gas whichis discharged from the storage tank T and then passes through theboil-off gas heat exchanger 112 to be supplied to the high pressureengine 180 via the third valve 193, the extra compressor 122, the extracooler 132, the fourth valve 194, the sixth valve 196. When it isnecessary to use the boil-off gas compressed by the extra compressor 122as the refrigerant of the refrigerant heat exchanger 140, the ninthvalve 201 and the thirteenth valve 205 may be open to operate thesystem.

When the refrigerant cycle of the ship of the present embodiment isoperated as the open loop and the liquefied gas stored in the storagetank T is liquefied natural gas and the high pressure engine 180 is theX-DF engine and the low pressure engine 181 is the DF engine, thecompressor 120 and the extra compressor 122 each compress the boil-offgas to approximately 17 bars which is the pressure required by the highpressure engine 180.

The boil-off gas at point A in FIG. 5, which is the confluent flow ofthe boil-off gas compressed by compressor 120 and the boil-offcompressed by extra compressor 122, may be approximately 43° C. and 17bars. The boil-off gas of approximately 43° C. and 17 bars may be cooledby the boil-off gas heat exchanger 112 to be approximately −110° C. and17 bars (C in FIG. 5), and the boil-off gas of approximately −100° C.and 17 bars may be expanded by the third decompressing device 301 to beapproximately −125° C. and 6.5 bars (D in FIG. 5).

The ship of the present embodiment may independently operate the extracompressor 122 and the compressor 120 while operating the refrigerantcycle as the open loop so that the boil-off gas compressed by the extracompressor 122 is used only as the refrigerant of the refrigerant heatexchanger 140, the boil-off gas compressed by the compressor 120 is sentto the high pressure engine 180 or undergoes the re-liquefaction processalong the return line L3 and is not used as the refrigerant of therefrigerant heat exchanger 140. Hereinafter, the refrigerant cycle ofthe open loop in which the extra compressor 122 and the compressor 120are operated independently is referred to as an ‘independent open loop’.

To operate the refrigerant cycle of the ship of the present embodimentas the independent open loop, the first valve 191, the second valve 192,the third valve 193, the fourth valve 194, the ninth valve 201, and thethirteenth valve 205 are open, and the sixth valve 196 and the tenthvalve 202 are closed. When the refrigerant cycle is operated as theindependent open loop, the system can be operated more easily than whenthe open loop is operated.

The flow of the boil-off gas in the case where the refrigerant cycle ofthe ship of the present embodiment is operated as the independent openloop will be described as follows.

The boil-off gas discharged from the storage tank T is branched into twoflows after passing through the boil-off gas heat exchanger 112 and apart thereof is sent to the first supply line L1 and the remaining partthereof is supplied to the second supply line L2. The boil-off gas sentto the first supply line L1 passes through the first valve 191, thecompressor 120, the cooler 130, and the second valve 192 and then a partthereof is sent to the high pressure engine 180 and the other partthereof is sent to the boil-off gas heat exchanger 112 along the returnline L3. The boil-off gas sent to the second supply line L2 passesthrough the third valve 193, the extra compressor 122, the extra cooler132, the fourth valve 194, and the thirteenth valve 205 and is then sentto the refrigerant heat exchanger 140.

The boil-off gas which is compressed by the extra compressor 122 andthen sent to the refrigerant heat exchanger 140 along the recirculationline L5 is used as the refrigerant which cools the boil-off gas which isprimarily heat-exchanged in the refrigerant heat exchanger 140 to becooled, secondarily expanded by the refrigerant decompressing device 160to be cooled, and then supplied back to the refrigerant heat exchanger140 to pass through the boil-off gas heat exchanger 112 and then besupplied to the refrigerant heat exchanger 140 via the return line L3and the boil-off gas which is compressed by the extra compressor 122 andthen supplied to the refrigerant heat exchanger 140 along therecirculation line L5.

The boil-off gas used as the refrigerant in the refrigerant heatexchanger 140 after passing through the refrigerant decompressing device160 is sent to the first supply line L1 through the ninth valve 201 tobe discharged from the storage tank T and then joins with the boil-offgas passing through the boil-off gas heat exchanger 112, therebyrepeating the above-mentioned series of processes.

Meanwhile, the boil-off gas, which is compressed by the compressor 120and sent to the heat exchanger 112 along the return line L3, is cooledby the boil-off gas heat exchanger 112 and then branched into two flows.

One flow of the boil-off gases, which are cooled by the boil-off gasheat exchanger 112 and then branched into two flows, is continuouslysupplied to the refrigerant heat exchanger 140 along the return line L3and heat-exchanged in the refrigerant heat exchanger 140 to beadditionally cooled. The boil-off gas cooled by the boil-off gas heatexchanger 112 and the refrigerant heat exchanger 140 is expanded by thefirst decompressing device 150 and thus the boil-off gas is partially ortotally re-liquefied.

When the ship of the present embodiment does not include the gas-liquidseparator 170, the boil-off gas partially or totally re-liquefied isdirectly sent to the storage tank T. When the ship of the presentembodiment includes the gas-liquid separator 170, the boil-off gaspartially or totally re-liquefied is sent to the gas-liquid separator170. The gas separated by the gas-liquid separator 170 is joined withthe boil-off gas discharged from the storage tank T and sent to theboil-off gas heat exchanger 112. The liquid separated by the gas-liquidseparator 170 is supplied to the storage tank T.

The other stream of the boil-off gases, which are cooled by the boil-offgas heat exchanger 112 and then branched into two flows, is expanded bythe third decompressing device 301 along the third supply line L7 andthen expanded back to the boil-off gas heat exchanger 112. The boil-offgas, which is expanded by the third decompressing device 301 and has thelowered pressure and temperature, is used as the refrigerant in theboil-off gas heat exchanger 112. The boil-off gas used as therefrigerant in the boil-off gas heat exchanger 112 after passing throughthe third decompressing device 301 is supplied to the low pressureengine 181 along the third supply line L7.

When the compressor 120 or the cooler 130 fails while the refrigerantcycle of the ship of the present embodiment is operated as theindependent closed loop, the first valve 191, the second valve 192, theninth valve 201, and the thirteenth valve 205 are closed and the sixthvalve 196 is open to allow the boil-off gas which is discharged from thestorage tank T and then passes through the boil-off gas heat exchanger112 to be supplied to the high pressure engine 180 via the third valve193, the extra compressor 122, the extra cooler 132, the fourth valve194, and the sixth valve 196. When it is necessary to use the boil-offgas compressed by the extra compressor 122 as the refrigerant of therefrigerant heat exchanger 140, the ninth valve 201 and the thirteenthvalve 205 may be open to operate the system.

FIG. 6 is a configuration diagram schematically showing a boil-off gastreatment system for a ship according to a fifth embodiment of thepresent invention.

The ship of the fifth embodiment shown in FIG. 6 is different from theship of the fourth embodiment shown in FIG. 5 in that the ship furtherincludes a boost compressor 124 which is installed on the return lineL3; and a boost cooler 134 which is installed on the downstream of theboost compressor 124 to increase the re-liquefaction efficiency and there-liquefaction amount in the boil-off gas heat exchanger 112 and therefrigerant heat exchanger 140. Hereinafter, the difference will bemostly described. The detailed description of the same member as theship of the foregoing fourth embodiment will be omitted.

Referring to FIG. 6, similar to the fourth embodiment, the ship of thepresent embodiment includes the boil-off gas heat exchanger 112, thefirst valve 191, the compressor 120, the cooler 130, the second valve192, the third valve 193, the extra compressor 122, the extra cooler132, the fourth valve 194, the refrigerant heat exchanger 140, therefrigerant decompressing device 160, the first decompressing device150, the third supply line L7, the ninth valve 201, the first additionalline L6, the tenth valve 202, and the thirteenth valve 205.

Similar to the fourth embodiment, the storage tank T stores liquefiedgas such as liquefied natural gas and liquefied ethane gas, anddischarges the boil-off gas to the outside when the internal pressure ofthe storage tank T exceeds a certain pressure or higher. The boil-offgas discharged from the storage tank (T) is sent to the boil-off gasheat exchanger 112.

Similar to the fourth embodiment, the boil-off gas heat exchanger 112 ofthe present embodiment uses, as the refrigerant, the boil-off gas whichis discharged from the storage tank T and the boil-off gas which issupplied along the third supply line L7 to cool the boil-off gas sent tothe boil-off gas heat exchanger 112 along the return line L3. Therefore,the boil-off gas heat exchanger 112 is a multiple heat exchanger.

Similar to the fourth embodiment, a fifth valve 195 which controls theflow rate of the boil-off gas and the opening and closing may beinstalled on the return line L3 and a twelfth valve 204 which controlsthe flow rate of the boil-off gas and the opening and closing may beinstalled on the third supply line L7 on the upstream of the lowpressure engine 181.

Similar to the fourth embodiment, according to the present embodiment,since not only the boil-off gas discharged from the storage tank T butalso the boil-off gas supplied along the third supply line L7 is used asthe refrigerant, the re-liquefaction efficiency and the re-liquefactionamount can more be increased than the third embodiment.

Similar to the fourth embodiment, the compressor 120 of the presentembodiment is installed on the first supply line L1 to compress theboil-off gas discharged from the storage tank T and similar to thefourth embodiment, the extra compressor 122 of the present embodiment isinstalled in parallel with the compressor 120 on the second supply lineL2 to compress the boil-off gas discharged from the storage tank T. Thecompressor 120 and the extra compressor 122 may be a compressor havingthe same performance, and each may be a multi-stage compressor.

Similar to the fourth embodiment, the compressor 120 and the extracompressor 122 of the present embodiment may compress the boil-off gasto a pressure required by the high pressure engine 180, compress theboil-off gas to a pressure higher than the pressure required by the highpressure engine 180, and the boil-off gas compressed to a pressurehigher than the pressure required by the high pressure engine 180 may bedecompressed to a pressure required by the high pressure engine 180 atthe front end of the high pressure engine 180 and then supplied to thehigh pressure engine 180.

Similar to the fourth embodiment, the ship of the present embodiment mayfurther include an eleventh valve 203 which is installed on the upstreamof the high-pressure engine 180 to control a flow rate of the boil-offgas sent to the high pressure engine 180 and the opening/closingthereof.

Similar to the fourth embodiment, the ship of the present embodimentuses the boil-off gas compressed by the extra compressor 122 as therefrigerant which additionally cools the boil-off gas in the refrigerantheat exchanger 140, thereby increasing the re-liquefaction efficiencyand the re-liquefaction amount.

Similar to the fourth embodiment, the cooler 130 of the presentembodiment is installed downstream of the compressor 120 to cool theboil-off gas that passes through the compressor 120 and has theincreased pressure and temperature. Similar to the fourth embodiment,the extra cooler 132 of the present embodiment is installed downstreamof the extra compressor 122 to cool the boil-off gas which passesthrough the extra compressor 122 and has the increased pressure andtemperature.

Similar to the fourth embodiment, the refrigerant heat exchanger 140 ofthe present embodiment additionally cools the boil-off gas which issupplied to the boil-off gas heat exchanger 112 along the return line L3and cooled by the boil-off gas heat exchanger 112.

Similar to the fourth embodiment, according to the present embodiment,the boil-off gas discharged from the storage tank T is further coolednot only in the boil-off gas heat exchanger 112 but also in therefrigerant heat exchanger 140, and may be supplied to the firstdecompressing device 150 in the state in which the temperature is lower,thereby increasing the re-liquefaction efficiency and there-liquefaction amount.

Similar to the fourth embodiment, the refrigerant decompressing device160 of the present embodiment expands the boil-off gas which passesthrough the refrigerant heat exchanger 140, and then sends the expandedboil-off gas back to the refrigerant heat exchanger 140.

Similar to the fourth embodiment, the first decompressing device 150 ofthe present embodiment is installed on the return line L3 to expand theboil-off gas cooled by the boil-off gas heat exchanger 112 and therefrigerant heat exchanger 140. The first decompressing device 150 ofthe present embodiment includes all means which may expand and cool theboil-off gas, and may be an expansion valve, such as a Joule-Thomsonvalve, or an expander.

Similar to the fourth embodiment, the ship of the present embodiment mayinclude the gas-liquid separator 170 which is installed on the returnline L3 on the downstream of the first decompressing device 150 andseparates the gas-liquid mixture discharged from the first decompressingdevice 150 into gas and liquid.

Similar to the fourth embodiment, when the ship of the presentembodiment does not include the gas-liquid separator 170, the liquid orthe boil-off gas in the gas-liquid mixed state which passes through thefirst decompressing device 150 is directly sent to the storage tank T,and when the ship of the present embodiment includes the gas-liquidseparator 170, the boil-off gas which passes through the firstdecompressing device 150 is sent to the gas-liquid separator 170 to beseparated into the gas phase and the liquid phase. The liquid separatedby the gas-liquid separator 170 returns to the storage tank T along thereturn line L3 and the gas separated by the gas-liquid separator 170 issupplied to the boil-off gas heat exchanger 112 along a gas dischargeline which extends from the gas-liquid separator 170 to the first supplyline L1 on the upstream of the boil-off gas heat exchanger 112.

Similar to the fourth embodiment, when the ship of the presentembodiment includes the gas-liquid separator 170, the ship may furtherinclude a seventh valve 197 which controls the flow rate of the liquidseparated by the gas-liquid separator 170 and sent to the storage tankT; an eighth valve 198 which controls the flow rate of gas separated bythe gas-liquid separator 170 and sent to the boil-off gas heat exchanger112.

Similar to the fourth embodiment, the third supply line L7 of thepresent embodiment is branched from the return line L3 between theboil-off gas heat exchanger 112 and the refrigerant heat exchanger 140and a part of the boil-off gas which is compressed by the compressor 120or the extra compressor 122 and then passes through the boil-off gasheat exchanger 112 along the return line L3 is sent back to the boil-offgas heat exchanger 112. The boil-off gas sent to the boil-off gas heatexchanger 112 along the third supply line L7 is used as a refrigerant inthe boil-off gas heat exchanger 112.

Similar to the fourth embodiment, the third decompressing device 301 ofthe present embodiment is installed on the third supply line L7 on theupstream of the boil-off gas heat exchanger 112 and expands a part ofthe boil-off gas which is cooled by the boil-off gas heat exchanger 112and then sent to the refrigerant heat exchanger 140 along the returnline L3.

Similar to the fourth embodiment, the third decompressing device 301 ofthe present embodiment includes all means which may expand and cool theboil-off gas, and may be an expansion valve, such as a Joule-Thomsonvalve, or an expander.

Similar to the fourth embodiment, one side of the first additional lineL6 of the present embodiment is connected to a recirculation line (notshown) which is expanded by the refrigerant decompressing device 160 andthen sent to the first supply line L1 through the refrigerant heatexchanger 140 L5 and the other side thereof is connected to the secondsupply line L2 between the third valve 193 and the extra compressor 122.

Similar to the fourth embodiment, the ninth valve 201 of the presentembodiment is installed on the recirculation line L5 between the pointwhere the recirculation line L5 meets the first supply line L1 on theupstream of the compressor 120 and the extra compressor 122 and thepoint where the recirculation line L5 meets the first additional lineL6, thereby controlling the flow rate of the fluid and theopening/closing thereof.

Similar to the fourth embodiment, the tenth valve 202 of the presentembodiment is installed on the first additional line L6 to control theflow rate of the fluid and the opening and closing thereof, and similarto the fourth embodiment, the thirteenth valve 205 of the presentembodiment is installed on the recirculation line L5 between the secondsupply line L2 and the refrigerant heat exchanger 140 to control theflow rate of the fluid and the opening and closing thereof.

Similar to the fourth embodiment, the first to thirteenth valves and theeleventh valve 191, 192, 193, 194, 195, 196, 197, 198, 201, 202, 203,204, and 205 of the present embodiment may be manually controlled byallowing a person to directly determine the operation condition of thesystem and may be automatically controlled to be opened or closeddepending on a preset value.

However, similar to the fourth embodiment, the ship of the presentembodiment is different from the third embodiment in that the shipfurther includes the boost compressor 124 which is installed on thereturn line L3 and the boost cooler 134 which is installed on the returnline L3 on the downstream of the boost compressor 124.

The boost compressor 124 of the present embodiment is installed on thereturn line L3 on which a part of the boil-off gas supplied to the highpressure engine 180 along the first supply line L1 is branched to besent to the boil-off gas heat exchanger 112, thereby increasing thepressure of the boil-off gas supplied to the boil-off gas heat exchanger112 along the return line L3. The boost compressor 124 may compress theboil-off gas to the pressure equal to or lower than a critical point (inthe case of methane, approximately 55 bars) or a pressure exceeding thecritical point, and the boost compressor 124 of the present embodimentmay compress the boil-off gas to approximately 300 bars if the boil-offgas is compressed to a pressure equal to or higher than the criticalpoint.

The boost cooler 134 of the present embodiment is installed on thereturn line L3 on the downstream of the boost compressor 124 to lowerthe boil-off gas which passes through the boost compressor 124 and hasnot only the reduced pressure but also the increased temperature.

The ship of the present embodiment further includes the boost compressor124 to increase the pressure of the boil-off gas undergoing there-liquefaction process, thereby increasing the re-liquefactionefficiency and the re-liquefaction amount.

FIGS. 8A and 8B are graphs showing temperature values of methanedepending on a heat flow under different pressures. Referring to FIGS.8A and 8B, it can be appreciated that the higher the pressure of theboil-off gas undergoing the re-liquefaction process, the higher theself-heat exchange efficiency becomes. The ‘self-’ of the self-heatexchange means the heat exchange with the high-temperature boil-off gasby the low-temperature boil-off gas itself as the cooling fluid.

FIG. 8A shows the state of each fluid in the refrigerant heat exchanger140 when the boost compressor 124 and the boost cooler 134 are notincluded, and FIG. 8B shows the state of each fluid in the refrigerantheat exchanger 124 when the boost compressor 124 and the boost cooler134 are included.

Graph I at the uppermost side in FIGS. 10A and 10B shows the fluid stateat point F in FIGS. 5 and 6 to which the refrigerant heat exchanger 140is supplied along the recirculation line L5, graph L at the lowest sideshows the fluid state of point G in FIGS. 5 and 6 which is supplied backto the refrigerant heat exchanger 140 to be used as the refrigerantafter passing through the refrigerant heat exchanger 140 and therefrigerant decompressing device 160 along the recirculation line L5,and graph J overlapping with graph K of an intermediate part shows thefluid state at point C in FIGS. 5 and 6 which is supplied to therefrigerant heat exchanger 140 along the return line L3 after passingthrough the boil-off heat exchanger 112.

Since the fluid used as the refrigerant is deprived of the cold heatduring the heat exchange process and the temperature thereof isgradually increased, the graph L proceeds from the left to the right astime passes, and since the fluid cooled by the heat exchange with therefrigerant is supplied with the cold heat from the refrigerant duringthe heat exchange process and the temperature thereof is reduced, thegraphs I and J proceed from the right to the left as time passes.

The graph K at the intermediate part of FIGS. 8A and 8B is shown by acombination of the graph I and the graph J. That is, the fluid used asthe refrigerant in the refrigerant heat exchanger 140 is drawn by thegraph L, and the fluid cooled by the heat exchange with the refrigerantin the refrigerant heat exchanger 140 is drawn by the graph K.

The heat exchanger is designed so that the temperature and the heat flowof the fluid supplied (i.e., the points F, G, and C in FIG. 5) to theheat exchanger may be fixed, the temperature of the fluid used as therefrigerant may not be higher than the temperature of the fluid to becooled (i.e., graphs L and K intersect each other so that the graph Ldoes not show above the graph K), and a logarithmic mean temperaturedifference (LMTD) may be as small as possible.

The logarithmic mean temperature difference (LMTD) is a valuerepresented by when in the case of a countercurrent flow which a heatexchanger manner in which the high-temperature fluid and thelow-temperature fluid are injected in an opposite direction to eachother and discharged from an opposite side from each other, thetemperature before the low-temperature fluid passes through the heatexchanger is tc1, the temperature after the low-temperature fluid passesthrough the heat exchanger is tc2, the temperature before thehigh-temperature fluid passes through the heat exchanger is th1, thetemperature after the high-temperature fluid passes through the heatexchanger is th2, and d1=th2−tc1 and d2=th1−tc2. The smaller thelogarithmic mean temperature difference, the higher the efficiency ofthe heat exchanger.

On the graph, the logarithmic mean temperature difference (LMTD) isrepresented by an interval between the low-temperature fluid (graph L inFIG. 8) used as the refrigerant and the high-temperature fluid (graph Kin FIG. 8) cooled by the heat exchange with the coolant. Here, it can beappreciated that the interval between the graph L and the graph K shownin FIG. 8B is narrower than the interval between the graph L and thegraph K shown in FIG. 8A.

The difference appears because an initial value of the graph J, which isa point represented by a round circle, that is, the pressure (FIG. 8B)of the point C in FIG. 6 which passes through the boil-off gas heatexchanger 112 and is then supplied to the refrigerant heat exchanger 140along the return line L3 is higher than the pressure (FIG. 8B) of thefluid of C point in FIG. 5.

That is, if the heat exchanger is designed so that the logarithmic meantemperature difference (LMTD) may be minimized under a certain initialcondition, in the case of FIG. 8B in which the pressure of the boil-offgas (point C in FIG. 6) undergoing the re-liquefaction process is high,the efficiency of the heat exchanger is further increased, and as aresult, the liquefaction amount and the liquefaction efficiency of theentire system are increased.

According to the present embodiment, it can be appreciated that if thepressure of the boil-off gas undergoing the re-liquefaction process,including the boost compressor 124 is increased, the larger amount ofboil-off gas may be re-liquefied even if a smaller amount of refrigerantis used.

As described, since the ship of the present embodiment includes theboost compressor 124, it is possible to increase the re-liquefactionamount and the re-liquefaction efficiency, and since the case in whichthe boil-off gas can be completely treated without operating the extracompressor 122 by increasing the re-liquefaction amount and there-liquefaction efficiency, the use frequency of the extra compressorcan be reduced.

Although the re-liquefaction efficiency can be increased by using theextra compressor 122, the longer the time to operate the extracompressor 122, the weaker the redundancy concept for preparing for thefailure of the compressor 120. The ship of the present embodiment canreduce the use frequency of the extra compressor 122 including the boostcompressor 124, and therefore the redundancy concept can be sufficientlysecured.

In addition, since the boost compressor 124 is generally sufficient tohave approximately one half capacity of the compressor 120 or the extracompressor 122, the operation cost may be more saved in the case inwhich the system is operated by operating only the boost compressor 124and the compressor 120 without operating the extra compressor 122 thanin the case in which the system is operated only by the compressor 120and the extra compressor 122 without the installation of the boostcompressor 124.

Similar to the fourth embodiment, the ship of the present embodiment mayoperate the refrigerant cycle not only as the open loop but also operateas the closed loop, and a method of operating a refrigerant cycle as aclosed loop, an open loop, and an independent open loop by a valvecontrol is similar to the fourth embodiment.

When the refrigerant cycle of the ship of the present embodiment isoperated as the open loop and the liquefied gas stored in the storagetank T is liquefied natural gas and the high pressure engine 180 is theX-DF engine and the low pressure engine 181 is the DF engine, thecompressor 120 and the extra compressor 122 each compress the boil-offgas to approximately 17 bars which is the pressure required by the highpressure engine 180.

The boil-off gas at point A in FIG. 6, which is the confluent flow ofthe boil-off gas compressed by compressor 120 and the boil-offcompressed by extra compressor 122, may be approximately 43° C. and 17bars. The boil-off gas of approximately 43° C. and 17 bars, may becompressed to be approximately 43° C. and 300 bars after beingcompressed by the boost compressor 124 and cooled by the boost cooler134 (B in FIG. 6).

The boil-off gas of approximately 43° C. and 300 bars may be cooled bythe boil-off gas heat exchanger 112 to be approximately −110° C. and 300bars (C in FIG. 6), and the boil-off gas of approximately −100° C. and300 bars may be expanded by the third decompressing device 301 to beapproximately −140° C. and 6.5 bars (D in FIG. 6).

When the boost compressor 124 compresses the boil-off gas toapproximately 300 bars, there is an effect that the re-liquefactionamount is increased by approximately 10% as compared with the fourthembodiment

The present invention is not limited to the above exemplary embodimentsand therefore it is apparent to a person with ordinary skill in the artto which the present invention pertains that the exemplary embodimentsof the present invention may be variously modified or changed withoutdeparting from the technical subjects of the present invention.

1. A ship including a storage tank storing liquefied gas, comprising: aboil-off gas heat exchanger which is installed on a downstream of astorage tank and heat-exchanges and cools a compressed boil-off gas(hereafter referred to as ‘a first fluid’) by a boil-off gas dischargedfrom the storage tank as a refrigerant; a compressor which is installedon a downstream of the boil-off gas heat exchanger and compresses a partof the boil-off gas discharged from the storage tank; an extracompressor which is installed on the downstream of the boil-off gas heatexchanger in parallel with the compressor and compresses the other partof the boil-off gas discharged from the storage tank; a refrigerant heatexchanger which additionally cools the first fluid which is cooled bythe boil-off gas heat exchanger; a refrigerant decompressing devicewhich expands a second fluid, which is sent to the refrigerant heatexchanger (a fluid sent to the refrigerant heat exchanger hereafterbeing referred to as ‘a second fluid’) and cooled by the refrigerantheat exchanger, and then sending same back to the refrigerant heatexchanger; a first decompressing device which is for expanding the firstfluid that is cooled by the boil-off gas heat exchanger and refrigerantheat exchanger; and a third decompressing device which expands a part ofthe first fluid which is cooled by the boil-off gas heat exchanger(hereafter referred to as “a third fluid”) and sends the expanded fluidback to the boil-off gas heat exchanger, wherein the boil-off gas heatexchanger uses the boil-off gas being discharged from the storage tankand the third fluid as a refrigerant and the refrigerant heat exchangerheat exchanges and cools both the first fluid and second fluid by theboil-off gas, which passes through the refrigerant decompressing device,as a refrigerant, the first fluid is either the boil-off gas which iscompressed by the compressor or a confluent flow of the boil-off gascompressed by the compressor and the boil-off gas compressed by theextra compressor, and the second fluid is either the boil-off gas whichis compressed by the extra compressor or a confluent flow of theboil-off gas compressed by the compressor and the boil-off gascompressed by the extra compressor.
 2. The ship of claim 1, furthercomprising: a gas-liquid separator that separates the partiallyre-liquefied liquefied gas passing through the boil-off gas heatexchanger, the refrigerant heat exchanger, and the first decompressingdevice and the boil-off gas remaining in a gas phase, wherein theliquefied gas separated by the gas-liquid separator is sent to thestorage tank, and the boil-off gas separated by the gas-liquid separatoris sent to the boil-off gas heat exchanger.
 3. The ship of claim 1,wherein the first fluid is branched into two flows on an upstream of ahigh pressure engine, and a part of the first fluid is sent to theboil-off gas heat exchanger to be cooled and the other part thereof issent to the high pressure engine.
 4. The ship of claim 1, furthercomprising: a fourth decompressing device which expands a third fluidused as a refrigerant in the boil-off gas heat exchanger, wherein theboil-off gas passing through the fourth decompressing device is sent toa low pressure engine
 5. The ship of claim 1, further comprising: aboost compressor which is installed on the upstream of the boil-off gasheat exchanger to compressor the first fluid supplied to the boil-offgas heat exchanger.
 6. The ship of claim 5, wherein the boost compressorhas a capacity of ½ relative to that of the compressor.
 7. The ship ofclaim 1, wherein the second fluid which is compressed by the extracompressor, passes through the refrigerant heat exchanger and therefrigerant decompressing device, and then used as the refrigerant ofthe refrigerant heat exchanger is sent back to the extra compressor toform a refrigerant cycle of a closed loop in which the extra compressor,the refrigerant heat exchanger, the refrigerant decompressing device,and the refrigerant heat exchanger are connected.
 8. The ship of claim1, wherein the second fluid which is compressed by the extra compressor,passes through the refrigerant heat exchanger and the refrigerantdecompressing device, and then used as the refrigerant of therefrigerant heat exchanger is discharged from the storage tank and then,joined with the boil-off gas passing the boil-off gas heat exchanger. 9.The ship of claim 1, comprising; a valve which is installed on a lineconnecting the boil-off gas compressed by the compressor with theboil-off gas compressed by the extra compressor, in which the valve isopen and closed to join the boil-off gas compressed by the compressorwith the boil-off gas compressed by the extra compressor or separate theboil-off gas compressed by the compressor from the boil-off gascompressed by the extra compressor.
 10. The ship of claim 1, wherein therefrigerant decompressing device is an expander, and the fluid justbefore passing through the refrigerant decompressing device and thefluid just after passing through the refrigerant decompressing device isa gas phase.
 11. A boil-off gas treatment system for a ship including astorage tank for storing a liquefied gas, comprising: a first supplyline along which boil-off gas, which is discharged from the storage tankand partially compressed by a compressor, is sent to a high pressureengine; a second supply line which is branched from the first supplyline and has an extra compressor provided thereon, with the extracompressor compressing the other part of the boil-off gas dischargedfrom the storage tank; a return line which is branched from the firstsupply line and has a boil-off gas heat exchanger, a refrigerant heatexchanger, and a first decompressing device provided thereon, with thecompressed boil-off gas being re-liquefied by passing through the boiloff gas heat exchanger, the refrigerant heat exchanger, and the firstdecompressing device; a recirculation line which has the refrigerantheat exchanger and a refrigerant decompressing device provided thereon,with the boil-off gas, which is cooled by passing through therefrigerant heat exchanger and a refrigerant decompressing device, beingsent back to the refrigerant heat exchanger to be used as a refrigerantand then joined with the boil-off gas discharged from the storage tank;a first additional line which connects between the recirculation line ona downstream of the refrigerant decompressing device and the refrigerantheat exchanger and a second supply line on an upstream, of the extracompressor; a third supply line which is branched from the return lineon the downstream of the boil-off gas heat exchanger, the boil-off gasbeing sent to the low pressure engine by passing through a thirddecompressing device, the boil-off gas heat exchanger and a fourthdecompressing device along the third supply line, wherein the boil-offgas heat exchanger heat-exchanges and cools the boil-off gas suppliedalong the return line by the boil-off gas discharged from the storagetank and the boil-off gas supplied along the third supply line as therefrigerant, and the refrigerant heat exchanger heat-exchanges and coolsboth of the boil-off gas supplied along the recirculation line and theboil-off gas supplied along the return line by the boil-off gas passingthrough the refrigerant decompressing device as the refrigerant.
 12. Theboil-off gas treatment system of claim 11, further comprising: a boostcompressor which is installed on the return line on the upstream of theboil-off gas heat exchanger to compress the boil-off gas.
 13. Theboil-off gas treatment system of claim 11, further comprising: a firstvalve which is installed on an upstream of the compressor on the firstsupply line; a second valve which is installed on a downstream of thecompressor on the first supply line; a third valve which is installed onan upstream of the extra compressor on the second supply line; a fourthvalve installed on a downstream of the extra compressor on the secondsupply line; a sixth valve which is provided between the first supplyline and the second supply line on the recirculation line along whichthe boil-off gas branched from the first supply line is sent to therefrigerant heat exchanger; a ninth valve which is installed on therecirculation line for sending the boil-off gas from the refrigerantheat exchanger to the first supply line; a tenth valve which isinstalled on the first additional line; and a thirteenth valve which isinstalled on the recirculation line between the second supply line andthe refrigerant heat exchanger.
 14. The boil-off gas treatment system ofclaim 13, wherein the system is operated while the third valve, thefourth valve, the twelfth valve, the fourteenth valve, and the fifteenthvalve are open and the second valve, the fifth valve, the sixth valve,the ninth valve, the tenth valve, and the thirteenth valve are closed,and if the boil-off gas is supplied to the compressor, the first valveis closed to form the refrigerant cycle of the closed loop in which theboil-off gas circulates the compressor, the fourteenth valve, therefrigerant heat exchanger, the refrigerant decompressing device, therefrigerant heat exchanger, and the twelfth valve.
 15. The boil-off gastreatment system of claim 13, wherein the first valve, the second valve,the third valve, the fourth valve, the sixth valve, the ninth valve, andthe thirteenth valve are open and the tenth valve is closed so that theboil-off gas compressed by the compressor and the boil-off gascompressed by the extra compressor is joined and operated.
 16. Theboil-off gas treatment system of claim 13, wherein the first valve, thesecond valve, the third valve, the fourth valve, the ninth valve, andthe thirteenth valve are open and the sixth valve and the tenth valve isclosed so that the boil-off gas compressed by the compressor and theboil-off gas compressed by the extra compressor are separated andoperated.
 17. A method including: branching boil-off gas discharged froma liquefied gas storage tank into two and compressing one flow of thebranched boil-off gas by a compressor; compressing the other flow by anextra compressor; sending at least, one of the boil-off gas compressedby the compressor and the boil-off gas compressed by the extracompressor to a high pressure engine or re-liquefying the boil-off gasto return the boil-off gas to the storage tank (hereinafter, referred toas ‘returning boil-off gas’) or re-circulate the boil-off gas(hereinafter, referred to as ‘re-circulated boil-off gas’),heat-exchanging and cooling the ‘returning boil-off gas’ by the boil-offgas discharged from the storage tank and the decompressed and cooledboil-off gas (hereinafter, referred to as ‘decompressed boil-off gas’)as a refrigerant, and then exchanging heat between the heat-exchangedand cooled ‘returning boil-off gas’ and the ‘re-circulated boil-off gas’to be additionally cooled; and cooling and expanding the ‘re-circulatedboil-off gas’ to exchange heat between the cooled and expanded‘re-circulated boil-off gas’ and the ‘returning boil-off gas’.
 18. Themethod of claim 17, wherein the ‘decompressed boil-off gas’ is boil-offgas decompressed after a part of the heat-exchanged and cooled‘returning boil-off gas’ is branched.
 19. The method of claim 17,wherein the ‘decompressed boil-off gas’ used as the refrigerant for theheat exchange is sent to the low pressure engine.
 20. The method ofclaim 17, wherein the downstream line of the compressor and thedownstream line of the extra compressor are connected to each other tojoin the boil-off gas compressed by the compressor with the boil-off gascompressed by the extra compressor.